During DNA replication, base mismatches occur with a certain frequency, either spontaneously or due to mutagenic influences. These are repaired by repair genes. Since primary mutations occur much more frequently than would be compatible with the cell's functional capacity, DNA repair genes are of extraordinary importance for maintaining the integrity of the genome.
People who inherit a defective allele of a repair gene as a germline mutation (in the case of a functional allele on the homologous chromosome) are more likely to suffer from certain types of tumours:
These include e.g. hereditary non-polyposis colon cancer
Or the recessively inherited hypersensitivity to light in Xeroderma pigmentosum.