Human colorectal, endometrial, and gastric cancers with defective DNA mismatch repair (MMR) have microsatellite instability, a unique molecular alteration characterized by widespread frameshift mutations of repetitive DNA sequences. We developed "Kangaroo," a bioinformatics program for searches in nucleotide and protein sequence databases, and performed an in silico genome scan for DNA coding microsatellites that may have novel mutations in MMR-deficient cancers. Examination of 29 previously untested coding polyadenines revealed widespread mutations in MMR-deficient colorectal cancers, with the highest frequencies in ERCC5, CASP8AP2, p72, RAD50, CDC25, RECQL1, CBF2, RACK7, GRK4, and DNAPK (range, 10-33%). This algorithm allows comprehensive mutation profiling of MMR-deficient cancers, an important step in understanding the pathogenesis of these neoplasms.