Maintaining the integrity of genomic DNA is an ongoing challenge for the cell’s DNA repair enzymes, and when DNA repair is faulty, cancer can develop. One such example is the human disease known as hereditary non-polyposis colorectal cancer (HNPCC), or Lynch syndrome. Lynch syndrome is characterized by the appearance of cancer at an early age due to mutations in the genes encoding the DNA repair enzymes responsible for mismatch repair. In many cases, the first cancers to arise in these individuals are colon cancers owing to mutations occurring in the rapidly dividing intestinal epithelial cells. Nucleotide mismatches can occur during DNA replication that must be repaired before the next cell division. If the damaged nucleotides and bases are not repaired, they can lead to diseases such as cancer. If the damage occurs in reproductive cells, it can cause transmissible, or heritable, genetic mutations. Mutations in genes encoding DNA repair enzymes can be catastrophic.