DNA repair genes are frequently repressed in cancers due to hypermethylation of CpG islands within their promoters.  In head and neck squamous cell carcinomas at least 15 DNA repair genes have frequently hypermethylated promoters; these genes are XRCC1, MLH3, PMS1, RAD51B, XRCC3, RAD54B, BRCA1, SHFM1, GEN1, FANCE, FAAP20, SPRTN, SETMAR, HUS1, and PER1.  About seventeen types of cancer are frequently deficient in one or more DNA repair genes due to hypermethylation of their promoters.  As summarized in one review article, promoter hypermethylation of the DNA repair gene MGMT occurs in 93% of bladder cancers, 88% of stomach cancers, 74% of thyroid cancers, 40%-90% of colorectal cancers and 50% of brain cancers.  Promoter hypermethylation of LIG4 occurs in 82% of colorectal cancers.  This review article also indicates promoter hypermethylation of NEIL1 occurs in 62% of head and neck cancers and in 42% of non-small-cell lung cancers; promoter hypermetylation of ATM occurs in 47% of non-small-cell lung cancers; promoter hypermethylation of MLH1 occurs in 48% of  squamous cell carcinomas; and promoter hypermethylation of FANCB occurs in 46% of head and neck cancers.
On the other hand, the promoters of two genes, PARP1 and FEN1, were hypomethylated and these genes were over-expressed in numerous cancers.  PARP1 and FEN1 are essential genes in the error-prone and mutagenic DNA repair pathway microhomology-mediated end joining.  If this pathway is over-expressed, the excess mutations it causes can lead to cancer.  PARP1 is over-expressed in tyrosine kinase-activated leukemias, in neuroblastoma, in testicular and other germ cell tumors, and in Ewing's sarcoma,  FEN1 is over-expressed in the majority of cancers of the breast, prostate, stomach, neuroblastomas, pancreatic, and lung.
DNA damage appears to be the primary underlying cause of cancer.  If accurate DNA repair is deficient, DNA damages tend to accumulate. Such excess DNA damage can increase mutational errors during DNA replication due to error-prone translesion synthesis. Excess DNA damage can also increase epigenetic alterations due to errors during DNA repair. Such mutations and epigenetic alterations can give rise to cancer (see malignant neoplasms).  Thus, CpG island hyper/hypo-methylation in the promoters of DNA repair genes are likely central to progression to cancer.