Chromosomal instability is one of the most important characteristics underlying tumorigenesis. Certain colorectal cancers with chromosomal instability have been shown to have inactivation of a spindle assembly checkpoint party due to mutation of Bub1, a mitotic checkpoint gene. We performed sequencing analysis on reverse transcriptase-polymerase chain reaction (RT-PCR) product of the Bub1 cDNA (entire coding sequence) from 8 human gastric cancer cell lines. In addition, genomic PCR products of Bub1 exon 8, 10, 12, 15 and kinase domain from 9 oral cancer cell lines were analyzed by polymerase chain reaction-single-stand conformation polymorphism (PCR-SSCP). Although sequencing analysis of the Bub1 cDNA revealed several point mutations in 8 gastric cancer cell lines, we could not confirm the mutations by analyzing genomic DNA. Furthermore, genomic PCR-SSCP analysis revealed no mutations in exon 8, 10, 12, 15 and kinase domain of the Bub1 gene in any oral cancer cell lines examined. These results suggest that mutation of the Bub1 gene might not play a role in human stomach and oral carcinogenesis.