Colon cancer is the third most common malignancy in men and women and ranks behind only lung cancer in cancer deaths. The most common disease predisposing patients to colorectal cancer is hereditary non-polyposis colon cancer (HNPCC). HNPCC stems from mutations in the DNA mismatch repair (MMR) genes. The MMR system corrects damaged DNA that results from errors during DNA replication or exposure to certain chemicals. Although many functional details of the MMR proteins have emerged, the mechanism by which flawed MMR contributes to tumorigenesis is not fully understood. Faulty MMR results in an elevated mutation rate (mutator phenotype). A mutator phenotype may increase the likelihood of tumorigenicity by increasing the chances of proto-oncogene and tumor suppressor mutations that ultimately cause cancer. More recent work has revealed that MMR proteins play an important role in cell-cycle arrest and apoptosis in response to certain DNA damaging agents. Thus, MMR mutations may affect tumorigenesis through multiple mechanisms. The central hypothesis we are testing in our lab is that multiple MMR functions are affected to different extents by cancer-associated mutations of MMR genes. A subset of HNPCC cases have been linked to missense mutations of the MMR genes hMSH2 and hMSH6. We have detailed the biochemical characteristics of seven hMSH2 missense mutations associated with HNPCC. We are currently examining the biochemical effects of cancer-associated missense mutations in hMSH6 as well. We will also examine the effects of these missense mutations on MMR cellular functions including DNA repair and cell-cycle checkpoint and apoptosis signaling. A thorough analysis of the biochemical and cellular MMR functions affected by cancer-causing mutations may provide important details for the development of diagnostic, preventive and therapeutic strategies.