Title of Invention: METHODS AND COMPOSITIONS FOR DIAGNOSIS AND TREATMENT OF CANCER Abstract: The present invention relates to the identification of nucleic acid and amino acid sequences that are characteristic of colorectal, in particular colonic, and gastric tumor tissues and colorectal, in particular colonic, and gastric tissues, and which represent targets for therapy or diagnosis of such tumor diseases in a subject. Detailed Description: Gastrointestinal cancer (cancer of the digestive system) includes cancers of the esophagus, gallbladder, liver, pancreas, stomach, small intestine, large intestine (colon) and rectum. Some form of gastrointestinal cancer amounts to about 20 percent of all newly diagnosed cancers every year. Colorectal cancer includes cancerous growths in the colon, rectum and appendix. It is the third most common form of cancer and the second leading cause of cancer-related death in the Western world. Together, colon and rectal cancers account for 10% of cancers in men and 11% of cancers in women. It is the second most common site-specific cancer affecting both men and women. Many colorectal cancers are thought to arise from adenomatous polyps in the colon. These mushroom-like growths are usually benign, but some may develop into cancer over time. Colorectal carcinoma often presents as metastatic disease that impedes curative surgery. Over 20% of patients present with metastatic (stage 1V) colorectal cancer at the time of diagnosis, and up to 25% of this group will have isolated liver metastasis that is potentially resectable. Cancer of the colon or colon cancer is a disease characterized by the development of malignant cells in the lining or epithelium of the first and longest portion of the large intestine. Malignant cells have lost normal control mechanisms governing growth. These cells may invade surrounding local tissue, or they may spread throughout the body and invade other organ systems. Colon cancer is thought to arise sporadically in about 80% of those who develop the disease. Twenty percent of people are thought to have genetic predisposition, meaning their genes carry a trigger for the disease. Development of colon cancer at an early age, or at multiple sites, or recurrent colon cancer, suggests a genetically transmitted form of the disease as opposed to the sporadic form. Screening of colorectal cancer involves physical examination, simple laboratory tests, and the visualization of the lining of the colon. To visualize the colon epithelium, clinicians use X rays (indirect visualization) and endoscopy (direct visualization). Proteins are sometimes produced by cancers, and these may be elevated in the patient's blood. When this occurs, the protein produced is known as a tumor marker. There is a tumor marker for some cancers of the colon; it is known as carcinoembryonic antigen, or CEA. Unfortunately, this protein may be made by other adenocarcinomas as well, or it may not be produced by a particular colon cancer. Therefore, screening by chemical analysis for CEA has not been helpful. CEA has been helpful when used in a follow-up role for patients treated for colon cancer if their tumor makes the protein. The treatment of colorectal cancer depends on the staging of the cancer. Colon cancer is assigned stages I through IV based on the following general criteria: Stage I: the tumor is confined to the epithelium or has not penetrated through the first layer of muscle in the bowel wall; Stage II: the tumor has penetrated through to the outer wall of the colon or has gone through it, possibly invading other local tissue; Stage III: any depth or size of tumor associated with regional lymph node involvement; Stage IV: any of previous criteria associated with distant metastasis. When colorectal cancer is caught at early stages (with little spread) it can be curable. However when it is detected at later stages (when distant metastases are present) it is less likely to be curable. Surgery remains the primary treatment while chemotherapy and/or radiotherapy may be recommended depending on the individual patient's staging and other medical factors. Surgery is used as primary therapy for stages I through III colon cancer unless there are signs that local invasion will not permit complete removal of the tumor, as may occur in advanced stage III tumors. However, this circumstance is rare, occurring in less than 2% of all colon cancer cases. Surgical treatment of colon cancer usually involves removal of the involved segment of colon (colectomy) along with its blood supply and regional lymph nodes. Usually, the partial colectomies are separated into right, left, transverse, or sigmoid sections based on the blood supply. The removal of the blood supply at its origin along with the regional lymph nodes that accompany it ensures an adequate margin of normal colon on either side of the primary tumor. When the cancer lies in a position such that the blood supply and lymph drainage between two of the major vessels, both vessels are taken to assure complete radical resection or removal (extended radical right or left colectomy). If the primary tumor penetrates through the bowel wall, any tissue adjacent to the tumor extension is also taken if feasible. Chemotherapy is useful for patients who have had all identifiable tumor removed and are at risk for recurrence (adjuvant chemotherapy). Chemotherapy may also be used when the cancer is stage 1V and is beyond the scope of regional therapy, but this use is rare. Adjuvant therapy is considered in stage II disease with deep penetration or in stage III patients. Standard therapy is treatment with 5-fluorouracil, (5FU) combined with leucovorin for a period of six to 12 months. 5FU is an antimetabolite, and leucovorin improves the response rate. (A response is a temporary regression of the cancer from chemotherapy.) Another agent, levamisole, (which seems to stimulate the immune system), may be substituted for leucovorin. These protocols reduce rate of recurrence by about 15% and reduce mortality by about 10%. The regimens do have some toxicity, but usually are tolerated fairly well. Similar chemotherapy may be administered for stage 1V disease or if a patient progresses and develops metastases. Results show response rates of about 20%. Unfortunately, these patients eventually succumb to the disease, and this chemotherapy may not prolong survival or improve quality of life in Stage 1V patients. Clinical trials have now shown that the results can be improved with the addition of another agent to this regimen. Irinotecan does not seem to increase toxicity but it improved response rates to 39%, added two to three months to disease-free survival, and prolonged overall survival by a little over two months. Radiation therapy is used as an adjunct to surgery if there is concern about potential for local recurrence post-operatively and the area of concern will tolerate the radiation. For instance, if the tumor invaded muscle of the abdominal wall but was not completely removed, this area would be considered for radiation. Radiation has significant dose limits when residual bowel is exposed to it because the small and large intestine do not tolerate radiation well. Radiation also is used in the treatment of patients with metastatic disease. It is particularly useful in shrinking metastatic colon cancer to the brain. Noncolorectal gastrointestinal cancers include pancreatic, hepatobiliary, gastric, small bowel, and esophageal cancer. Treatment of noncolorectal gastrointestinal cancers often includes a multimodal approach combining surgery, radiotherapy, and chemotherapy. Cancer of the stomach—also called gastric cancer or gastric carcinoma—is a treatable disease that can often be cured when it is found and treated at a local stage. Stomach cancers are classified according to what sort of tissues they start in. The most common type arises in the glandular tissue lining the stomach. These tumors are called adenocarcinomas and account for over 95 percent of all stomach tumors. Metastases through the bloodstream can spread to the liver, lungs, bone and brain. Metastases are also found in the lining of the abdominal cavity (peritoneum) and around the rectum. The ideal treatment of stomach cancer is radical surgery, meaning that most or all of the stomach is removed (subtotal or total gastrectomy), along with the surrounding lymph nodes. Radical surgery is the only treatment that can lead to a cure, though lesser surgical procedures can play a significant role in therapy designed to relieve symptoms. Radiation and chemotherapy are also treatment options. The immune system has the ability to recognize and destroy cells via two separate modalities: innate and adaptive immunity. The innate component consists of macrophages, natural killer (NK) cells, monocytes, and granulocytes. These cells identify molecular patterns involved in cellular transformation and release various cytokines and inflammatory mediators. The innate response lacks the memory capability for foreign antigens, a feature present in adaptive immune response. This latter component of immune system also features specificity for foreign antigens, imparted by presence of receptors on lymphocytes. Antigen presenting cells (APCs) also play a role in the adaptive response—they engulf foreign antigens and present them to the lymphocytes in the context of major histocompatibility complex. CD4+ T cells bear receptors that recognize antigens in the context of MHC class II molecules, which then enables them to release cytokines and further activate CD8+ lymphocytes (CTLs) or B cells. CTLs are part of cell-mediated immunity and are capable of eliminating cells presented in the context of MHC class I molecules, via apoptosis or perforin-mediated cell lysis. It is widely accepted that T-cell mediated immunity plays a vital role in the anti-tumor response. B cells are involved in release of immunoglobulins and as such are part of the humoral immune system. If properly aimed and enhanced, immune functions can be therapeutically exploited to control and even eradicate malignant lesions. Genetic and epigenetic changes involved in carcinogenesis generate antigens that are recognized by the immune system in analogous fashion to microbial antigens. Novel strategies for the treatment of colorectal cancer such as active immunization with dendritic cells (DCs), gene transfer of cytokines into tumor cells or administration of immunostimulatory monoclonal antibodies have been assessed in preclinical studies and are at an early clinical development stage. Importantly, there is accumulating evidence that chemotherapy and immunotherapy can be combined in the treatment of some cases with colorectal cancer, with synergistic potentiation as a result of antigens cross-presented by dendritic cells and/or elimination of competitor or suppressive T lymphocyte populations (regulatory T-cells). There is a need in the art for genetic markers and targets of colorectal tumors, in particular colon tumors, and gastric tumors and metastatic tumors derived therefrom, allowing the design of specific, reliable and sensitive diagnostic and therapeutic approaches of these diseases. The invention relates to the therapy and diagnosis of colorectal tumors, in particular colon tumors, and stomach tumors and metastatic tumors derived therefrom. In particular, the invention relates to the identification of molecular structures that are present on colorectal tumors and gastric tumors and can serve as targets for diagnostic and therapeutic approaches of these diseases. SUMMARY OF THE INVENTION The present invention relates to the identification of nucleic acid and amino acid sequences that are characteristic of colorectal, in particular colonic, and gastric tumor tissues and colorectal, in particular colonic, and gastric tissues, and which represent targets for therapy or diagnosis of such tumor diseases in a subject. These sequences encompass proteins identified to be in the plasma membrane of the cells, and accessible on the extra-cellular region, so that the sequences may be useful in the preparation of tumor vaccines, including prophylatic and therapeutic vaccines. The nucleic acids identified according to the invention to be expressed in tumor cells comprise the nucleic acid sequence according to SEQ ID NO: 1 of the sequence listing or a variant of said nucleic acid sequence. Preferably, the nucleic acids identified according to the invention to be expressed in tumor cells encode a peptide comprising the amino acid sequence according to SEQ ID NO: 2 of the sequence listing or a variant of said amino acid sequence. These nucleic acids are also termed “tumor-associated nucleic acids” or simply “tumor nucleic acids” herein. In another aspect, the invention relates to peptides encoded by the tumor nucleic acids identified according to the invention, also termed “tumor-associated antigens” or simply “tumor antigens” herein. Accordingly, the tumor antigens identified according to the invention comprise an amino acid sequence encoded by a nucleic acid which comprises the nucleic acid sequence according to SEQ ID NO: 1 of the sequence listing or a variant of said nucleic acid sequence. Preferably, the tumor antigens identified according to the invention comprise the amino acid sequence according to SEQ ID NO: 2 of the sequence listing or a variant of said amino acid sequence. In one aspect, the invention provides peptides comprising amino acid sequences derived from the sequences of the tumor antigens identified according to the invention, also termed “tumor antigen peptides” herein. Preferably, the tumor antigen peptides of the invention are capable of stimulating a cellular response against cells characterized by presentation of a tumor antigen identified according to the invention with class I MHC and/or of elicting antibodies that specifically bind to a tumor antigen identified according to the invention when used itself or attached to an immunogenic carrier. Preferred tumor antigen peptides may be presented, directly or following processing, with class I MHC molecules. Preferably, the tumor antigen peptides according to the invention are MHC class I and/or class II presented peptides or can be processed to produce MHC class I and/or class II presented peptides. Preferably, the tumor antigen peptides according to the invention comprise an amino acid sequence substantially corresponding to the amino acid sequence of a fragment of a tumor antigen identified according to the invention. Preferably, said fragment of a tumor antigen identified according to the invention is a MHC class I and/or class II presented peptide or is an immunogen that is capable of elicting antibodies binding to said fragment. Preferably, a tumor antigen peptide according to the invention comprises an amino acid sequence substantially corresponding to the amino acid sequence of such fragment and is processed to produce such fragment, i.e. a MHC class I and/or class II presented peptide derived from a tumor antigen identified according to the invention or an immunogen derived from a tumor antigen identified according to the invention that is capable of elicting antibodies binding to said fragment. Thus, a tumor antigen peptide according to the invention comprises an amino acid sequence substantially corresponding to the amino acid sequence of a fragment of a tumor antigen comprising an amino acid sequence encoded by a nucleic acid which comprises the nucleic acid sequence according to SEQ ID NO: 1 of the sequence listing or a variant of said nucleic acid sequence and preferably comprises an amino acid sequence substantially corresponding to the amino acid sequence of a fragment of a tumor antigen comprising the amino acid sequence according to SEQ ID NO: 2 of the sequence listing or a variant of said amino acid sequence. The present invention generally embraces the treatment and/or diagnosis of tumor diseases by targeting tumor nucleic acids or tumor antigens. These methods provide for the selective detection of cells and/or eradication of cells that express such tumor nucleic acids and/or tumor antigens thereby minimizing adverse effects to normal cells not expressing such tumor nucleic acids and/or tumor antigens. Thus, preferred diseases for a therapy or diagnosis are those in which one or more of the tumor nucleic acids and/or tumor antigens identified according to the invention are expressed such as tumor diseases, in particular cancer diseases such as those described herein. According to the invention, particularly suitable for targeting the tumor antigens identified according to the invention is a part of the tumor antigens which corresponds to the non-transmembrane portion, in particular the extracellular portion of the tumor antigens or is comprised thereof. Therefore, the entities used according to the invention which are capable of binding to the tumor antigens identified according to the present invention preferably are capable of binding to a part of the tumor antigens identified according to the invention which corresponds to the non-transmembrane portion, in particular the extracellular portion of the tumor antigens or is comprised thereof. Similarly, peptides and nucleic acids used according to the invention for inducing an immune response with specificity to the tumor antigens identified according to the present invention preferably induce specificity for a part of the tumor antigens identified according to the invention which corresponds to the non-transmembrane portion, in particular the extracellular portion of the tumor antigens or is comprised thereof. Preferably, said peptides comprise a sequence substantially corresponding to a part of the tumor antigens identified according to the invention which corresponds to the non-transmembrane portion, in particular the extracellular portion of the tumor antigens or is comprised thereof, and said nucleic acids encode such peptides. One aspect of this invention relates to therapies for treatment of tumor diseases, in particular colorectal tumors such as colon tumors and gastric tumors, involving the administration of an inhibitor of expression and/or activity of a tumor antigen identified according to the invention. In this aspect, the present invention relates to a pharmaceutical composition comprising an inhibitor of expression and/or activity of a tumor antigen identified according to the invention. In one embodiment, said inhibitor is specific for a tumor nucleic acid identified according to the invention. In another embodiment, said inhibitor is specific for a tumor antigen identified according to the invention. According to the invention the phrase “inhibit expression and/or activity” includes a complete or essentially complete inhibition of expression and/or activity and a reduction in expression and/or activity. Preferably, said inhibition of expression of a tumor antigen identified according to the invention may take place by inhibiting the production of or reducing the level of transcript, i.e. mRNA, coding for a tumor antigen identified according to the invention, e.g. by inhibiting transcription or inducing degradation of transcript, and/or by inhibiting the production of tumor antigen identified according to the invention, e.g. by inhibiting translation of transcript coding for a tumor antigen identified according to the invention. Preferably, said inhibition of expression and/or activity of a tumor antigen identified according to the present invention reduces tumor cell growth and/or induces tumor cell death and thus, has a tumor-inhibiting or tumor-destroying effect. In a particular embodiment, the inhibitor of expression of a tumor antigen identified according to the invention is an inhibitory nucleic acid (e.g., anti-sense oligonucleotide, ribozyme, iRNA, siRNA or a DNA encoding the same) selectively hybridizing to and being specific for a tumor nucleic acid identified according to the invention, thereby inhibiting (e.g., reducing) transcription and/or translation thereof. Inhibitory nucleic acids of this invention include oligonucleotides having sequences in the antisense orientation relative to the target nucleic acids. Suitable inhibitory oligonucleotides typically vary in length from five to several hundred nucleotides, more typically about 20-70 nucleotides in length or shorter, even more typically about 10-30 nucleotides in length. These inhibitory oligonucleotides may be administered as free (naked) nucleic acids or in protected forms, e.g., encapsulated in liposomes. The use of liposomal or other protected forms may be advantageous as it may enhance in vivo stability and thus facilitate delivery to target sites. Also, the target tumor nucleic acid may be used to design ribozymes that target the cleavage of the corresponding mRNAs in tumor cells. Similarly, these ribozymes may be administered in free (naked) form or by the use of delivery systems that enhance stability and/or targeting, e.g., liposomes. Also, the target tumor nucleic acid may be used to design siRNAs that can inhibit (e.g., reduce) expression of the tumor nucleic acid. The siRNAs may be administered in free (naked) form or by the use of delivery systems that enhance stability and/or targeting, e.g., liposomes. They may also be administered in the form of their precursors or encoding DNAs. siRNA preferably comprises a sense RNA strand and an antisense RNA strand, wherein the sense and antisense RNA strands form an RNA duplex, and wherein the sense RNA strand comprises a nucleotide sequence substantially identical to a target sequence of about 19 to about 25 contiguous nucleotides in a tumor nucleic acid identified according to the invention, preferably mRNA coding for the target tumor antigen. In a further embodiment, the inhibitor of activity of a tumor antigen identified according to the invention is an antibody that specifically binds to said tumor antigen. Binding of the antibody to the tumor antigen can interfere with the function of the tumor antigen, e.g. by inhibiting binding activity or catalytic activity. Also, the present invention in another aspect relates to therapies for treatment of tumor diseases involving the administration of a ligand of a target molecule, i.e. a tumor nucleic acid or tumor antigen identified according to the invention. In this respect, a nucleic acid may be administered that selectively hybridizes to the target nucleic acid or an antibody may be administered that specifically binds to a target antigen, attached to therapeutic effector moieties, e.g., radiolabels, cytotoxins, cytotoxic enzymes, and the like in order to selectively target and kill cells that express these targets, e.g. tumor cells. In this aspect, the present invention relates to a pharmaceutical composition, comprising a ligand of a tumor nucleic acid or tumor antigen identified according to the invention, said ligand being attached to one or more therapeutic effector moieties. Preferably, said ligand is specific for said tumor nucleic acid or tumor antigen. In one embodiment, said ligand of a tumor nucleic acid or tumor antigen reduces tumor cell growth and/or induces tumor cell death and thus, has a tumor-inhibiting or tumor-destroying effect. According to a further aspect of the invention, the identification of tumor nucleic acids and tumor antigens makes it possible to develop specific immunotherapies based on attacking tumor cells bearing the identified antigens, thereby delaying or preventing the development of a tumor disease or eradicating tumor cells. Immunotherapy encompasses a variety of interventions and techniques with the common goal of eliciting tumor cell destructive immune responses. A variety of clinical approaches utilising these nucleic acids and antigens are possible as summarised below. Approaches to cancer immunotherapy can be divided into active and passive categories. Active immunotherapy may involve the direct immunization of patients with antigens or nucleic acids encoding such antigens in an attempt to boost immune responses against the tumor. Passive immunotherapy refers to the administration of immune reagents with the goal of directly mediating antitumor responses. The present invention contemplates both approaches. In this aspect, the invention relates to a pharmaceutical composition which comprises one or more agents selected from the group consisting of (i) a peptide comprising the amino acid sequence of a tumor antigen identified according to the invention or of a tumor antigen peptide derived from said tumor antigen, or a derivative of said peptide, (ii) a nucleic acid which codes for a peptide comprising the amino acid sequence of a tumor antigen identified according to the invention or of a tumor antigen peptide derived from said tumor antigen, or a derivative of said nucleic acid, (iii) a host cell which codes for a peptide comprising the amino acid sequence of a tumor antigen identified according to the invention or of a tumor antigen peptide derived from said tumor antigen, (iv) a virus which codes for a peptide comprising the amino acid sequence of a tumor antigen identified according to the invention or of a tumor antigen peptide derived from said tumor antigen, (v) a cell presenting a peptide comprising the amino acid sequence of a tumor antigen peptide derived from a tumor antigen identified according to the invention, or a derivative of said peptide, (vi) an antibody or T cell receptor which binds to a peptide comprising the amino acid sequence of a tumor antigen identified according to the invention or of a tumor antigen peptide derived from said tumor antigen, (vii) an immunoreactive cell sensitized in vitro to recognize a peptide comprising the amino acid sequence of a tumor antigen identified according to the invention or of a tumor antigen peptide derived from said tumor antigen, and (viii) an effector cell (or stem cell) transduced with a nucleic acid encoding a T cell receptor that recognises a peptide comprising the amino acid sequence of a tumor antigen identified according to the invention or of a tumor antigen peptide derived from said tumor antigen. In one embodiment, a peptide according to (i) is a tumor antigen specific MHC class I or class II presented peptide or can be processed to produce a tumor antigen specific MHC class I or class II presented peptide, preferably a tumor antigen specific MHC class I presented peptide. Preferably, said peptide has a sequence substantially corresponding to a fragment of a tumor antigen identified according to the invention which is presented by MHC class I or class II, preferably MHC class I or can be processed to produce a peptide fragment having such sequence. Preferably, said peptide is capable of stimulating a cellular response against a tumor characterized by presentation of a tumor antigen identified according to the invention with class I MHC and/or is capable of stimulating a humoral immune response against a tumor characterized by expression of a tumor antigen identified according to the invention. In one embodiment, a nucleic acid according to (ii) codes for a tumor antigen specific MHC class I or class II presented peptide or codes for a peptide which can be processed to produce a tumor antigen specific MHC class I or class II presented peptide, preferably a tumor antigen specific MHC class I presented peptide. Preferably, said peptide has a sequence substantially corresponding to a fragment of a tumor antigen identified according to the invention which is presented by MHC class I or class II, preferably MHC class I or can be processed to produce a peptide fragment having such sequence. Preferably, said peptide is capable of stimulating a cellular response against a tumor characterized by presentation of a tumor antigen identified according to the invention with class I MHC and/or is capable of stimulating a humoral immune response against a tumor characterized by expression of a tumor antigen identified according to the invention. Such nucleic acid may be present in a plasmid or an expression vector and may be functionally linked to a promoter. In one embodiment, a host cell according to (iii) codes for a tumor antigen specific MHC class I or class II presented peptide or codes for a peptide which can be processed to produce a tumor antigen specific MHC class I or class II presented peptide, preferably a tumor antigen specific MHC class I presented peptide. Preferably, said peptide has a sequence substantially corresponding to a fragment of a tumor antigen identified according to the invention which is presented by MHC class I or class II, preferably MHC class I or can be processed to produce a peptide fragment having such sequence. Preferably, said peptide is capable of stimulating a cellular response against a tumor characterized by presentation of a tumor antigen identified according to the invention with class I MHC and/or is capable of stimulating a humoral immune response against a tumor characterized by expression of a tumor antigen identified according to the invention. The host cell may be a recombinant cell and may secrete the encoded peptide or a procession product thereof, may express it on the surface and preferably may additionally express an MHC molecule which binds to said peptide or a procession product thereof and preferably presents said peptide or a procession product thereof on the cell surface. In one embodiment, the host cell expresses the MHC molecule endogenously. In a further embodiment, the host cell expresses the MHC molecule and/or the peptide or the procession product thereof in a recombinant manner. The host cell is preferably nonproliferative. In a preferred embodiment, the host cell is an antigen-presenting cell, in particular a dendritic cell, a monocyte or a macrophage. In one embodiment, a virus according to (iv) codes for a tumor antigen specific MHC class I or class II presented peptide or codes for a peptide which can be processed to produce a tumor antigen specific MHC class I or class II presented peptide, preferably a tumor antigen specific MHC class I presented peptide. Preferably, said peptide has a sequence substantially corresponding to a fragment of a tumor antigen identified according to the invention which is presented by MHC class I or class II, preferably MHC class I or can be processed to produce a peptide fragment having such sequence. Preferably, said peptide is capable of stimulating a cellular response against a tumor characterized by presentation of a tumor antigen identified according to the invention with class I MHC and/or is capable of stimulating a humoral immune response against a tumor characterized by expression of a tumor antigen identified according to the invention. In one embodiment, a cell according to (v) endogenously expresses an MHC molecule. In a further embodiment, the cell recombinantly expresses an MHC molecule and/or a peptide comprising the amino acid sequence of a tumor antigen peptide derived from a tumor antigen identified according to the invention. Preferably, the cell presents the peptide comprising the amino acid sequence of a tumor antigen peptide derived from a tumor antigen identified according to the invention, or a derivative of said peptide by MHC molecules on its surface. Preferably, the presented peptide is a peptide having a sequence substantially corresponding to a fragment of a tumor antigen identified according to the invention which is presented by MHC class I or class II, preferably MHC class I. The cell is preferably nonproliferative. In a preferred embodiment, the cell is an antigen-presenting cell such as a dendritic cell, a monocyte or a macrophage. Thus, in a preferred embodiment, the cell according to (v) is an antigen presenting cell that comprises a tumor antigen peptide as described herein presented with class I MHC. In one embodiment, an antibody according to (vi) is a monoclonal antibody. In further embodiments, the antibody is a chimeric, human or humanized antibody, or is a fragment of an antibody or a synthetic antibody. The antibody may be coupled to a therapeutic effector moiety or a detectable label. Preferably, the antibody or T cell receptor according to (vi) binds to a sequence in the peptide substantially corresponding to a fragment of a tumor antigen identified according to the invention. Preferably, a cell according to (vii) binds to a sequence in the peptide substantially corresponding to a fragment of a tumor antigen identified according to the invention which fragment is preferably presented by MHC class I or class II, preferably MHC class I. In one embodiment, a cell according to (vii) is obtainable by a method comprising the steps of (a) providing a sample containing immunoreactive cells, either obtained from a patient or from another individual of the same species, in particular a healthy individual, or an individual of a different species, (b) contacting said sample with cells presenting a peptide comprising an amino acid sequence substantially corresponding to a fragment of a tumor antigen identified according to the invention, or a derivative of said peptide, under conditions which favor production of CTLs against said peptide, and (c) introducing the CTLs into the patient in an amount suitable for lysing cells expressing the tumor antigen and preferably presenting it with class I MHC such as tumor cells. In one embodiment, the method includes cloning of the T cell receptor of CTLs obtained and transferring the nucleic acid coding for the T cell receptor to effector cells such as CTLs or immature CTLs, either obtained from said patient or from another individual of the same species, in particular a healthy individual, or an individual of a different species, which effector cells thus receive the desired specificity and may be introduced into the patient. Effector cells according to (viii) can be produced in this way. Vaccination using agents as described above may provide MHC class II-presented epitopes that are capable of eliciting a CD4+ helper T-cell response and/or a CD8+ T-cell response against tumor antigens identified according to the invention, in particular if expressed in cells such as tumor cells. Alternatively or additionally, vaccination using agents as described above may provide MHC class I-presented epitopes that are capable of eliciting a CD8+ T-cell response against tumor antigens identified according to the invention, in particular if expressed in cells such as tumor cells. Furthermore, vaccination using agents as described above may elicit antibodies specific for a tumor antigen identified according to the invention. In one embodiment, the pharmaceutical composition of the present invention is a therapeutic or prophylactic anti-tumor vaccine preferably further comprising an immunomodulatory agent, or a nucleic acid encoding the same. In one embodiment, the immunomodulatory agent is an agonist of a positive costimulatory molecule, e.g., an Ig-fusion protein capable of effecting costimulation of a CTL. In another embodiment, the costimulatory agent is an antagonist of a negative costimulatory molecule, e.g., an antibody capable of reducing inhibition of CTL costimulation. In a preferred embodiment, the immunomodulatory agent is an anti-CTLA4 antibody. A pharmaceutical composition of the invention may comprise a pharmaceutically acceptable carrier and may optionally comprise one or more adjuvants, stabilizers etc. Another aspect of the invention involves the use of the agents and compositions described herein for a prophylactic and/or therapeutic treatment of tumor diseases. In one aspect, the invention provides therapeutic and prophylactic methods of treating a patient having a tumor disease or being at risk of developing a tumor disease. In one aspect, the invention provides methods for inhibiting tumor growth. In one aspect, the invention provides methods for inducing tumor cell death. Preferably, the tumor disease is a cancer disease, preferably selected from the group consisting of colorectal cancer, in particular colon cancer, gastric cancer, metastatic colorectal cancer, in particular colon cancer, and metastatic gastric cancer. In one embodiment, the tumor cell is a cell of such a cancer. According to various embodiments, the methods of the invention comprise the administration of an inhibitor of expression and/or activity of a tumor antigen identified according to the invention, of a ligand of a tumor nucleic acid or of a tumor antigen identified according to the invention and/or of one or more immunotherapeutic agents as described herein. In one embodiment, the methods involve administering a pharmaceutical composition as described herein to a patient and preferably vaccinating a patient with an anti-tumor vaccine described herein. Any of the wide variety of vaccination methods known in the art may be used according to the present invention. Anti-tumor vaccines of the invention are preferably capable of inducing or promoting CTL activity against a tumor characterized by presentation of a tumor antigen identified according to the invention with class I MHC. These may be used in combination with adjuvants, which facilitate stimulation of the immune system by acting on T cells directly or through APCs. Adjuvants include immunomodulatory substances having a positive immunomodulatory effect, as described herein. In various embodiment, the methods of the invention involve the stimulation of an anti-tumor CTL response against tumor cells expressing a tumor antigen identified according to the invention and preferably presenting a tumor antigen identified according to the invention with class I MHC, the inhibition of the growth of tumor cells expressing a tumor antigen identified according to the invention and preferably presenting a tumor antigen identified according to the invention with class I MHC, and/or the induction of the death of cells expressing a tumor antigen identified according to the invention and preferably presenting a tumor antigen identified according to the invention with class I MHC. In one aspect, the invention provides an inhibitor of expression and/or activity of a tumor antigen identified according to the invention, a ligand of a tumor nucleic acid or of a tumor antigen identified according to the invention and/or one or more immunotherapeutic agents as described herein for use in the methods of treatment described herein. In one embodiment, the invention provides a pharmaceutical composition as described herein for use in the methods of treatment described herein. The treatments based on targeting tumor nucleic acids or tumor antigens such as the immunotherapies described herein can be combined with surgical resection and/or radiation and/or traditional chemotherapy. Another object of the invention is to provide methods for diagnosis, detection or monitoring, i.e. determining the regression, progression, course and/or onset, of a tumor disease. Preferably said methods involve the use of ligands such as monoclonal antibodies and nucleic acid probes which specifically bind to a target molecule. Suitable target molecules are (i) a tumor nucleic acid identified according to the invention, (ii) a tumor antigen identified according to the invention or a tumor antigen peptide derived therefrom, (iii) an antibody against a tumor antigen identified according to the invention or a tumor antigen peptide derived therefrom, (iv) a T cell which recognizes a tumor antigen identified according to the invention or a tumor antigen peptide derived therefrom and/or (v) a cell which presents a tumor antigen peptide derived from a tumor antigen identified according to the invention with class I or class II MHC, preferably class I MHC. Such methods may be used to detect whether a subject has or is at (increased) risk of developing a tumor disease, or, for instance, whether a treatment regimen is efficient. Accordingly, the present invention relates to methods for diagnosis, detection or monitoring of a tumor disease comprising the detection of and/or determination of the quantity of one or more parameters selected from the group consisting of (i) a nucleic acid which comprising the nucleic acid sequence of a tumor nucleic acid identified according to the invention/a nucleic acid which codes for a peptide comprising the amino acid sequence of a tumor antigen identified according to the invention, (ii) a peptide comprising the amino acid sequence of a tumor antigen identified according to the invention or of a tumor antigen peptide derived from said tumor antigen, (iii) an antibody which binds to a peptide comprising the amino acid sequence of a tumor antigen identified according to the invention or of a tumor antigen peptide derived from said tumor antigen, (iv) a T cell that recognises a peptide comprising the amino acid sequence of a tumor antigen identified according to the invention or of a tumor antigen peptide derived from said tumor antigen and/or (v) a cell which presents a peptide comprising the amino acid sequence of a tumor antigen peptide derived from a tumor antigen identified according to the invention with class I or class II MHC, preferably class I MHC, in a biological sample isolated from a patient, preferably from a patient having a tumor disease, being suspected of having or falling ill with a tumor disease or having a potential for a tumor disease. In one embodiment, a nucleic acid according to (i) codes for a peptide which is processed to produce a tumor antigen specific MHC class I or class II presented peptide, preferably, a tumor antigen specific MHC class I presented peptide. In one embodiment, a peptide according to (ii) is a tumor antigen specific MHC class I or class II presented peptide or can be processed to produce a tumor antigen specific MHC class I or class II presented peptide, preferably, a tumor antigen specific MHC class I presented peptide. Preferably, a T cell according to (iv) recognizes a sequence in the peptide substantially corresponding to a fragment of a tumor antigen identified according to the invention which is presented by MHC class I or class II, preferably MHC class I. In one embodiment, a cell according to (v) presents the peptide by MHC class I or class II, preferably MHC class I on its surface. The cell is preferably nonproliferative. In a preferred embodiment, the cell is an antigen-presenting cell such as a dendritic cell, a monocyte or a macrophage. Thus, in a preferred embodiment, the cell according to (v) is an antigen presenting cell that comprises a tumor antigen peptide as described herein presented with class I MHC. In another embodiment, the cell is a tumor cell. In one embodiment, the nucleic acid according to (i) or the peptide according to (ii) is detected or its quantity determined in situ in a cell, preferably a tumor cell. In one embodiment, the peptide according to (ii) is detected or its quantity determined in situ on the surface of a cell, either incorporated in the plasma membrane or in a complex with MHC class I or class II, preferably MHC class I. In one embodiment of the methods for diagnosis, detection or monitoring of a tumor disease the biological sample is from a tissue or organ wherein the cells when the tissue or organ is free of tumors do not substantially express a tumor antigen identified according to the invention and/or a tumor nucleic acid identified according to the invention. Preferably said tissue is a tissue other than colon tissue or gastric tissue. Preferably, said tissue is tissue of small intestine, brain, breast, liver, lung, pancreas, kidney, prostate, spleen, lymph node, endometrium, esophagus, placenta, ovary, testis, uterus, skin, thymus, bladder, muscle and cervix. According to the invention a tumor antigen and/or a tumor nucleic acid is not substantially expressed if the level of expression is lower compared to expression in colon cells and/or stomach cells or colon tissue and/or stomach tissue. Preferably, the level of expression is less than 10%, preferably less than 5%, 3%, 2%, 1%, 0.5%, 0.1% or 0.05% or even lower compared to colon cells and/or stomach cells or colon tissue and/or stomach tissue. Preferably, a tumor antigen and/or a nucleic acid is not substantially expressed if the level of expression is below the detection limit. The methods for diagnosis, detection or monitoring allow quantitative and/or qualitative evaluations, e.g., absolute and/or relative measure of target molecules e.g. expression levels of a tumor nucleic acid or a tumor antigen. Means for accomplishing said detection and/or determination of the quantity are described herein and will be apparent to the skilled person. Preferably, the detection and/or determination of the quantity in the methods of the invention comprises (i) contacting a biological sample with an agent which binds specifically to the nucleic acid, the peptide, the antibody, the T cell or the cell which is to be detected and/or the amount of which is to be determined, and (ii) detecting the formation of and/or determining the quantity of a complex between the agent and the nucleic acid, the peptide, the antibody, the T cell or the cell which is to be detected or the amount of which is to be determined. Typically, the level of a target molecule in a biological sample is compared to a reference level, wherein a deviation from said reference level is indicative of the presence and/or stage of a tumor disease in a subject. The reference level may be a level as determined in a control sample (e.g., from a healthy tissue or subject) or a median level from healthy subjects. A “deviation” from said reference level designates any significant change, such as an increase or decrease by at least 10%, 20%, or 30%, preferably by at least 40% or 50%, or even more. Preferably, the presence of the nucleic acid, the peptide, the antibody, the T cell and/or the cell in said biological sample or a quantity of the nucleic acid, the peptide, the antibody, the T cell and/or the cell in the biological sample which is increased compared to a reference level indicates the presence of a tumor disease. Typically, the detection and/or determination of the quantity in the methods of the invention involves the use of labeled ligands which specifically bind to a target molecule, e.g. a labeled nucleic acid probe that hybridizes to a target nucleic acid and/or a labeled antibody or fragment/derivative thereof that specifically binds to a target peptide. According to the invention, detection of a nucleic acid or determining the quantity of a nucleic acid may be carried out using known nucleic acid detection methods such as methods involving hybridization or nucleic acid amplification techniques. In one embodiment, mRNA transcripts are detected or the quantity thereof is determined using RT-PCR or Northern blot analysis. Such nucleic acid detection methods may involve the use of oligonucleotides hybridizing to the target nucleic acids. Suitable oligonucleotides typically vary in length from five to several hundred nucleotides, more typically about 20-70 nucleotides in length or shorter, even more typically about 10-30 nucleotides in length. According to the invention, detection of a peptide or determining the quantity of a peptide may be carried out in a number of ways, including but not limited to immunodetection using an antibody binding specifically to said peptide. Preferably, the antibody binds to a sequence substantially corresponding to a fragment of a tumor antigen identified according to the invention. Methods for using antibodies to detect peptides are well known and include ELISA, competitive binding assays, and the like. In general, such assays use an antibody or antibody fragment that specifically binds the target peptide directly or indirectly bound to a label that provides for detection, e.g. indicator enzymes, radiolabels, fluorophores, or paramagnetic particles. According to the invention, detection of an antibody or determining the quantity of an antibody may be carried out using a peptide binding specifically to said antibody. T cells may be isolated from patient peripheral blood, lymph nodes, tissue samples such as derived from biopsy and resection, or other source. Reactivity assays may be performed on primary T cells or other appropriate derivatives. For example, T cells may be fused to generate hybridomas. Assays for measuring T cell responsiveness are known in the art, and include proliferation assays and cytokine release assays. In one embodiment, the T cell that recognises a peptide comprising the amino acid sequence of a tumor antigen identified according to the invention or of a tumor antigen peptide derived from said tumor antigen is a tumor antigen-responsive CTL. A CTL may be detected and its quantity determined in a number of ways, including but not limited to the following preferred embodiments. In one embodiment, CTLs are directly stained using an appropriate fluorescent tumor antigen peptide/MHC tetramer. In another embodiment, the “TRAP” assay (“T-cell recognition of APCs by protein transfer”) is used (see, for example, Beadling et al. Nature Medicine 12:1208 (2006)). In another embodiment, detection of T cells in blood samples is performed using methods outlined by Yuan et al. (Cytotherapy 8:498, 2006). Assays and indices for detecting reactive T cells are known, and include but are not limited to the use of IFN-gamma ELISPOT and IFN-gamma intracellular cytokine staining. Other various methods are known in the art for determining whether a T cell clone will respond to a particular antigenic peptide. Typically the peptide is added to a suspension of the T cells for a period of from one to three days. The response of the T cells may be measured by proliferation, e.g., uptake of labeled thymidine, or by release of cytokines, e.g., IL-2. Various assays are available for detecting the presence of released cytokines. T cell cytotoxic assays can be used to detect cytotoxic T cells having specificity for tumor antigens. In one embodiment, cytotoxic T cells are tested for their ability to kill target cells presenting tumor antigen peptide with MHC class I molecules. Target cells presenting tumor antigen peptide may be labeled and added to a suspension of T cells from a patient sample. The cytotoxicity may be measured by quantitating the release of label from lysed cells. Controls for spontaneous and total release may be included in the assay. A cell presenting a peptide may be detected and its quantity determined by testing for its ability to induce a cellular response, e.g. to activate T cells, or measuring lysis of cells by CTLs having specificity for such cell. The presence of said nucleic acid, said peptide, said antibody, said T cell and/or said cell which is to be detected and/or the quantity of which is to be determined and/or a quantity of said nucleic acid, said peptide, said antibody, said T cell and/or said cell which is increased compared to a reference level, e.g. compared to a patient without a tumor disease, may indicate the presence of or risk for (i.e. a potential for a development of) a tumor disease in said patient. In one embodiment, the presence of said nucleic acid, said peptide, said antibody, said T cell and/or said cell which is to be detected and/or the quantity of which is to be determined and/or a quantity of said nucleic acid, said peptide, said antibody, said T cell and/or said cell which is increased compared to a reference level, e.g. compared to a patient without a tumor disease, may indicate the presence of or risk for metastatic colorectal cancer, in particular metastatic colon cancer, or metastatic gastric cancer in said patient. In one embodiment, the biological sample is from a tissue or organ wherein the cells when the tissue or organ is free of tumors do not substantially express a tumor antigen identified according to the invention and/or a tumor nucleic acid identified according to the invention. The indication of the presence of or risk for a tumor disease in a patient by the methods of the invention may
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