Lynch syndrome, or hereditary nonpolyposis colon cancer (HNPCC), is an 
autosomal-dominant disease accounting for approximately 1-5% of all colorectal 
cancer cases. Due to the lack of pathognomonic morphological or biomolecular 
markers, HNPCC has traditionally posed unique problems to clinicians and 
geneticists alike, both in terms of diagnosis and clinical management. Recently, 
novel insight into the pathogenesis of this syndrome has been provided by the 
identification of its molecular basis. In HNPCC families, germline mutations in 
any of four genes encoding proteins of a specialized DNA repair system, the 
mismatch repair, predispose to cancer development. Mutations in mismatch repair 
genes lead to an overall increase of the mutation rate and are associated with a 
phenotype of length instability of microsatellite loci. The present report 
summarizes the clinicopathological aspects of HNPCC and reviews the most recent 
molecular and biochemical findings.