Conversion of diploidy to haploidy is a method that allows the generation of stable murine/human hybrid cell lines carrying selected human chromosomes in only a single copy. In this setting, it is possible to detect genetic mutations with greater sensitivity and reliability than in diploid cells. Using this method, we were able to identify mutations in the human mismatch repair (MMR) gene hMSH2 in hereditary nonpolyposis colon cancer families, which have escaped detection by the conventional methods. In this report, we show that such hybrid cell lines can also be a valuable tool in the study of the mutated MMR proteins, in particular the variants found in hereditary nonpolyposis colon cancer families that carry missense mutations and where it is unclear whether they predispose to colon cancer. This analysis is made possible by the fact that the human hMSH2 protein is able to complement the MMR defect in the host murine cell line.