Hereditary non-polyposis colorectal cancer (HNPCC) is one of the most common conditions predisposing to colorectal cancer (CRC). Affecting less than 1% of the general population, HNPCC confers a lifetime risk of CRC estimated at over 80%; the risk to age 40 may exceed 30%.1 It can be caused by inherited mutations in one of the several DNA mismatch repair (MMR) genes. The resulting DNA MMR deficiency in neoplastic tissue gives rise to microsatellite alterations, a reflection of genomic instability.2Patients with HNPCC are at increased risk of cancers other than CRC, including cancers of the endometrium, ovary, stomach, small bowel, and the upper urinary tract (renal pelvis and ureter).3

Despite recent advances in the understanding of the molecular genetic basis of HNPCC, we are still far from the clinical ideal of being able to identify patients with this disorder among the population we serve. Tests for mutations in MMR genes are problematic. There are classic HNPCC families with clear linkage to one of the MMR genes which have been exhaustively but fruitlessly searched for mutations, indicating some types of HNPCC associated mutations are not identifiable by current technologies. Even when an inherited alteration of an MMR gene is discovered, its causal …