Biochemical and genetic approaches have been used to demonstrate that basic elements of a DNA mismatch repair (MMR) pathway are conserved between bacteria, yeast and mammals. Recently, mutations in the human MMR genes MSH2, MLH1, PMS1 and PMS2 have been implicated in a common form of hereditary colon cancer and in sporadic tumors of various tissues. In order to better understand the consequences of MMR deficiency in mammalian organisms, mice deficient for the Pms2, Mlh1 and Msh2 MMR gene homologues have been generated. MMR deficient mice display a general increase in spontaneous mutation rate and develop tumors during the first year of life. Additionally, loss of MMR appears to accelerate tumorigenesis in an Apc deficient background.