can also give rise to epigenetic alterations during DNA repair. Both mutations and epigenetic alterations (epimutations) can contribute to progression to cancer. Very frequent mutations in cancer As noted above, about 3 or 4 driver mutations and 60 passenger mutations occur in the exome (protein coding region) of a cancer. However, a much larger number of mutations occur in the non-protein-coding regions of DNA. The average number of DNA sequence mutations in the entire genome of a breast cancer tissue sample is about 20,000. In an average melanoma tissue sample (where melanomas have a higher exome mutation frequency) the total number of DNA sequence mutations is about 80,000. Cause of high frequency of mutations in cancer The high frequency of mutations in the total genome within cancers suggests that, often, an early carcinogenic alteration may be a deficiency in DNA repair. Mutation rates substantially increase (sometimes by 100-fold) in cells defective in DNA mismatch repair or in homologous recombinational DNA repair. Also, chromosomal rearrangements and aneuploidy increase in humans defective in DNA repair gene BLM. A deficiency in DNA repair, itself, can allow DNA damages to accumulate, and error-prone translesion synthesis past some of those damages may give rise to mutations. In addition, faulty repair of these accumulated DNA damages may give rise to epigenetic alterations or epimutations. While a mutation or epimutation in a DNA repair gene, itself, would not confer a selective advantage, such a repair defect may be carried along as a passenger in a cell when the cell acquires an additional mutation/epimutation that does provide a proliferative advantage. Such cells, with both proliferative advantages and one or more DNA repair defects (causing a very high mutation rate), likely give rise to the 20,000 to 80,000 total genome mutations frequently seen in cancers. DNA repair deficiency in cancer In somatic cells, deficiencies in DNA repair sometimes arise by mutations in DNA repair genes, but much more often are due to epigenetic reductions in expression of DNA repair genes. Thus, in a sequence of 113 colorectal cancers, only four had somatic missense mutations in the DNA repair gene MGMT, while the majority of these cancers had reduced MGMT expression due to methylation of the MGMT promoter region. Five reports, listed in the article Epigenetics (see section "DNA repair epigenetics in cancer") presented evidence that between 40% and 90% of colorectal cancers have reduced MGMT expression due to methylation