Chromosomal instability in colorectal cancers is associated with functional loss of a mitotic check point partly due to mutations of the Bub1, one of the mitotic check point genes. However, mutation of coding sequences of human Bub1 gene has not been fully elucidated in gastric carcinomas. We performed sequencing analysis on reverse transcriptase-polymerase chain reaction (RT-PCR) product of the Bub1 cDNA (entire coding sequence) from 5 human gastric carcinomas as well as on genomic PCR products of Bub1 kinase domain from 7 gastric carcinoma tissues. Although sequencing analysis of the Bub1 cDNA revealed several point mutations in 2 gastric carcinoma cases, we could not confirm the mutations by analyzing genomic DNA. Furthermore, genomic DNA sequencing revealed no mutations in the kinase domain of the Bub1 gene in any gastric carcinoma examined. These results suggest that mutational inactivation of the Bub1 gene might not play a key role in human stomach carcinogenesis.