A rapid increase in our understanding of the basic mechanisms involved in the genesis of colorectal cancer has occurred. Important factors include the activation of oncogenes, the inactivation of tumor suppressor genes and the inactivation of DNA repair systems. Hereditary non-polyposis colorectal cancer (HNPCC) has been linked to germ-line mutations that inactivate one allele of a DNA mismatch repair gene, which pre-disposes affected individuals to a high frequency of early-onset cancer. We have developed a cellular model to study HNPCC using the DNA mismatch repair-deficient colon cancer cell line HCT116 and corrected the genetic deficiency by chromosome transfer. Human chromosome 3 corrected all manifestations of DNA mismatch repair deficiency in the cell line and restored sensitivity to the toxic effects of certain DNA alkylating agents. Data suggest that the DNA mismatch repair genes not only participate in the recognition and repair of DNA mismatches during new strand synthesis but may also signal cell cycle arrest in the face of overwhelming DNA damage. Additional functions for the DNA mismatch repair genes in the maintenance of DNA replicative fidelity are being studied.