tumour development is the failure to repair damaged DNA

Another mechanism for tumour development is the failure
to repair damaged DNA. Xeroderma pigmentosum, for
example, is a rare autosomal recessive disorder caused by
failure to repair DNA damaged by ultraviolet light. Exposure to
sunlight causes multiple skin tumours in affected individuals.
Many other tumours are found to be associated with instability
of multiple microsatellite markers because of a failure to repair
mutated DNA containing mismatched base pairs. Microsatellite
instability is particularly common in colorectal, gastric and
endometrial cancers. Hereditary non-polyposis colon cancer
(HNPCC) is due to mutations in genes on chromosomes 2p,
2q, 3p and 7p. The hMSH2 gene on chromosome 2p represents
a mismatch repair gene. Some patients with HNPCC inherit
one mutant copy of this gene, which is inactivated in all cells.
Loss of the other allele (loss of heterozygosity) in colonic cells
leads to an increase in the mutation rate in other genes,
resulting in the development of colonic cancer.