Inactivation of the DNA mismatch repair (MMR) system allows the genome to accumulate mutations because of failure to correct mispairing of nucleotides and slippage mistakes at microsatellite sequences (termed microsatellite instability ¿MSI¿). While most mutations are acquired in noncoding regions by virtue of its larger share of DNA, mutations may occur in exons of genes that contain microsatellite sequences. The type II receptor for TGF beta 1 (TGF beta RII), the insulin-like growth factor II receptor (IGFIIR), and the proapoptotic gene BAX have been shown to contain mononucleotide microsatellites, and in MSI tumors, mutations may occur in these sequences late in the multistep carcinogenesis pathway. Here, we characterize 9 cell lines for MSI and mutations in TGF beta RII, BAX, and IGFIIR by PCR-based assays. The MMR-proficient cell lines SW480 and HT29 demonstrate stability at microsatellite sequences and do not have mutations in TGF beta RII, BAX, or IGFIIR. The MMR-deficient cell lines LoVo, SW48, LS174t, and HCT116 all demonstrate MSI and have only mutant alleles of TGF beta RII. Additionally, SW48 cells were heterozygous for wild-type and mutant IGFIIR. While LoVo and LS174t cells possessed only mutant BAX alleles, HCT116 was heterozygous and SW48 had only the wild-type allele. The MMR-deficient ovarian cell line 2774 demonstrated MSI, but showed only wild-type TGF beta RII, IGFIIR, and BAX alleles. In HCT116+ch2 cells, there was no genotypic change from the hMLH1-mutated HCT116 cells. In HCT116+ch3 cells, MSI was corrected, and this cell line became heterozygous for mutant and wild-type TGF beta RII because wild-type hMLH1 and TGF beta RII are both located on chromosome 3. Thus, the presence of a defective MMR system correlates with MSI, and the wild-type allele of TGF beta RII was absent in all microsatellite unstable colon cell lines, whereas the absence of wild-type BAX occurred in only two colon cell lines. In ovarian cancer cells with MSI, mutations in TGF beta RII, BAX, and IGFIIR may be unimportant in the genesis of this tumor.