Familial colorectal cancer type X: the other half of hereditary nonpolyposis colon cancer syndrome.

Establishing the Amsterdam criteria, based on pedigrees, was essential for defining hereditary nonpolyposis colorectal cancer (HNPCC) syndrome in such a way that the underlying genetic cause could be identified. It is now known that about half of families that fulfill the original Amsterdam criteria have a hereditary DNA mismatch repair (MMR) gene mutation. These families may be said to have Lynch syndrome. The other half of families with HNPCC has no evidence of DNA MMR deficiency, and studies show that these families are different from families with Lynch syndrome. Familial colorectal cancer type X is the name used to refer to the "other half of HNPCC".