HNPCC is an autosomal dominant disease that is clinically characterized by the development of colorectal cancer (CRC) at an early age (mean age 44 years old). Four genes have been known to be related to this hereditary disease. It shows an excess of synchronous and metachronous tumors as well as a preponderance of right-sided tumors (70%). Another feature has been seen among the families of the HNPCC patients is the occurrence of adenocarcinomas at other sites (particularly at the endometrial, ovary, stomach, pancreas, ureter, renal pelvis, and skin). Difficulties arise in distinguishing environmental factors and genetic predisposition for familial clustering of CRC. The discovery of HNPCC germline mutations has been momentous in that it enables a clear distinction between carriers and noncarriers for those who were previously assigned a 50% risk of germline mutation. The informed consent provided by patients is important for the process of familial study and the search for germline mutations, these will further provide information for education and counseling. HNPCC has been reported to be responsible for about 1% to 13% of all CRC. The frequency of HNPCC varies by geographical areas. The true incidence of HNPCC in Taiwan area is unclear. From year 1995 to 2000, 50 out of 4500(1.1%) patients were HNPCC according to the Amsterdam I criteria. MMR gene databases are crucial to understand the relationship between genotype and phenotype. Kindred sharing the same mutations but living in different places will provide the information to assess the contribution of environmental factors to colorectal carcinogenesis. The related clinical and basic researches are thus important for understanding the mutation spectrum of MMR genes, interaction between oncogenes, tumor suppressor genes, and roles of genetic polymorphisms in modifying MMR genes in Taiwan.