During the last 15 years, several single-gene mendelian disorders have been discovered that might account for some of the familial aggregation detected in large population studies of colorectal cancer (CRC). Mutations in DNA mismatch repair (MMR) genes cause hereditary nonpolyposis colorectal cancer-Lynch syndrome, the most common of the recognized CRC-predisposition syndromes, in which one major feature is a young age for cancer onset