Mutations in DNA mismatch repair (MMR) genes have been shown to segregate with Hereditary Nonpolyposis Colorectal Cancer (HNPCC). However, because many HNPCC families fail to display mutations in known MMR genes, we argued that changes in other components of the MMR pathway may be responsible. The increasing number of proteins reported to interact in the MMR pathway suggests that larger complexes are formed, the composition of which may differ among cell types and tissues. In an attempt to identify tissue-specific MMR-associated factors, we employed the yeast two-hybrid system, using the human hMSH2 as bait and a human fetal liver library as prey. We demonstrate that hMSH2 interacts with a human 5'-3' exonuclease 1 (hEXO1/HEX1) and that this interaction is mediated through their C-terminal domains. The hMSH6 protein does not interact with hEXO1 in the two-hybrid system. Dot-blot analysis of multiple tissue RNA revealed that hMSH2 and hEXO1 are coexpressed at high levels in fetal liver as well as in adult testis and thymus. Northern blot analysis also revealed that hEXO1/HEX1 is highly expressed in several liver cancer cell lines as well as in colon and pancreas adenocarcinomas, but not in the corresponding non-neoplastic tissue.