impossible to show in IHC that the staining corresponds with the protein of interest. For this reason, primary antibodies must be well-validated in a Western Blot or similar procedure. The technique is even more widely used in diagnostic surgical pathology for immunophenotyping tumors (e.g. immunostaining for e-cadherin to differentiate between DCIS (ductal carcinoma in situ: stains positive) and LCIS (lobular carcinoma in situ: does not stain positive)). More recently, Immunohistochemical techniques have been useful in differential diagnoses of multiple forms of salivary gland, head, and neck carcinomas. The diversity of IHC markers used in diagnostic surgical pathology is substantial. Many clinical laboratories in tertiary hospitals will have menus of over 200 antibodies used as diagnostic, prognostic and predictive biomarkers. Examples of some commonly used markers include: BrdU: used to identify replicating cells. Used to identify tumors as well as in neuroscience research. Cytokeratins: used for identification of carcinomas but may also be expressed in some sarcomas. CD15 and CD30 : used for Hodgkin's disease. Alpha fetoprotein: for yolk sac tumors and hepatocellular carcinoma. CD117 (KIT): for gastrointestinal stromal tumors (GIST) and mast cell tumors. CD10 (CALLA): for renal cell carcinoma and acute lymphoblastic leukemia. Prostate specific antigen (PSA): for prostate cancer. estrogens and progesterone receptor (ER & PR) staining are used both diagnostically (breast and gyn tumors) as well as prognostic in breast cancer and predictive of response to therapy (estrogen receptor). Identification of B-cell lymphomas using CD20. Identification of T-cell lymphomas using CD3. PIN-4 cocktail, targeting p63, CK-5, CK-14 and AMACR (latter also known as P504S), and used to distinguish prostate adenocarcinoma from benign glands. Directing therapy A variety of molecular pathways are altered in cancer and some of the alterations can be targeted in cancer therapy. Immunohistochemistry can be used to assess which tumors are likely to respond to therapy, by detecting the presence or elevated levels of the molecular target. Chemical inhibitors Tumor biology allows for a number of potential intracellular targets. Many tumors are hormone dependent. The presence of hormone receptors can be used to determine if a tumor is potentially responsive to antihormonal therapy. One of the first therapies was the antiestrogen, tamoxifen, used to treat breast cancer. Such hormone receptors can be detected by immunohistochemistry. Imatinib, an intracellular tyrosine kinase inhibitor, was developed to treat chronic myelogenous leukemia, a disease characterized by the formation of a specific abnormal tyrosine kinase. Imitanib has proven effective