THE human DNA mismatch repair gene homologue, hMSH2, on chromosome 2p is involved in hereditary non-polyposis colon cancer (HNPCC)^1,2. On the basis of linkage data, a second HNPCC locus was assigned to chromosome 3p21-23 (ref. 3). Here we report that a human gene encoding a protein, hMLHl (human MutL homologue), homologous to the bacterial DNA mismatch repair protein MutL, is located on human chromosome 3p21.3-23. We propose that hMLHl is the HNPCC gene located on 3p because of the similarity of the hMLHl gene product to the yeast DNA mismatch repair protein, MLH1^4,5, the coincident location of the hMLHl gene and the HNPCC locus on chromosome 3, and hMLHl missense mutations in affected individuals from a chromosome 3-linked HNPCC family.