Hereditary nonpolyposis colorectal cancer (HNPCC) is an autosomal dominant disease caused by mutations in one of at least four different DNA mismatch repair genes, hMLH1, hMSH2, hPMS1, and hPMS2. Phenotypically, HNPCC is characterized by the early onset of colorectal cancers and various extracolonic cancers. Depending on the presence or absence of extracolonic tumors, HNPCG-has been divided into two syndromes (Lynch syndrome I and Lynch syndrome II), but, so far, no correlation to distinct genotypes has been demonstrated. In this study, we present a frequent hMLH1 intron 14 founder mutation that is associated with a highly reduced frequency of extracolonic tumors. The mutation disrupts the splice donor site and silences the mutated allele. Tumors exhibited microsatellite instability, and loss of the wild-type hMLH1 allele was prevalent. We propose that the mutation results in a milder phenotype, because the mutated hMLH1 protein is prevented from exerting a dominant negative effect on the concerted action of the mismatch repair system.