Defects in the repair systems cause cancer Besides base excision repair, nucleotide excision repair, and mismatch repair, there are several other mechanisms that maintain our DNA. Every day, they fix thousands of occurrences of DNA damage caused by the sun, cigarette smoke or other genotoxic substances; they continuously counteract spontaneous alterations to DNA and, for each cell division, mismatch repair corrects some thousand mismatches. Our genome would collapse without these repair mechanisms. If just one component fails, the genetic information changes rapidly and the risk of cancer increases. Congenital damage to the nucleotide excision repair process causes