of the MGMT promoter region. Similarly, for 119 cases of colorectal cancers classified as mismatch repair deficient and lacking DNA repair gene PMS2 expression, Pms2 was deficient in 6 due to mutations in the PMS2 gene, while in 103 cases PMS2 expression was deficient because its pairing partner MLH1 was repressed due to promoter methylation (PMS2 protein is unstable in the absence of MLH1). The other 10 cases of loss of PMS2 expression were likely due to epigenetic overexpression of the microRNA, miR-155, which down-regulates MLH1. In cancer epigenetics (see section Frequencies of epimutations in DNA repair genes), there is a partial listing of epigenetic deficiencies found in DNA repair genes in sporadic cancers. These include frequencies of between 13–100% of epigenetic defects in genes BRCA1, WRN, FANCB, FANCF, MGMT, MLH1, MSH2, MSH4, ERCC1, XPF, NEIL1 and ATM located in cancers including breast, ovarian, colorectal and head and neck. Two or three epigenetic deficiencies in expression of ERCC1, XPF and/or PMS2 were found to occur simultaneously in the majority of the 49 colon cancers evaluated. Some of these DNA repair deficiencies can be caused by epimutations in microRNAs as summarized in the MicroRNA article section titled miRNA, DNA repair and cancer. Lymphomas as a consequence of genome instability Cancers usually result from disruption of a tumor repressor or dysregulation of an oncogene. Knowing that B-cells experience DNA breaks during development can give insight to the genome of lymphomas. Many types of lymphoma are caused by chromosomal translocation, which can arise from breaks in DNA, leading to incorrect joining. In Burkitt’s lymphoma, c-myc, an oncogene encoding a transcription factor, is translocated to a position after the promoter of the immunoglobulin gene, leading to dysregulation of c-myc transcription. Since immunoglobulins are essential to a lymphocyte and highly expressed to increase detection of antigens, c-myc is then also highly expressed, leading to transcription of its targets, which are involved in cell proliferation. Mantle cell lymphoma is characterized by fusion of cyclin D1 to the immunoglobulin locus. Cyclin D1 inhibits Rb, a tumor suppressor, leading to tumorigenesis. Follicular lymphoma results from the translocation of the immunoglobulin promoter to the Bcl-2 gene, giving rise to high levels of Bcl-2 protein, which inhibits apoptosis. DNA-damaged B-cells no longer undergo apoptosis, leading to further mutations which could affect driver genes, leading to tumorigenesis. The location of translocation in the oncogene shares structural properties of the target regions