hBUB1 gene is a component of the mitotic checkpoint that monitors proper assembly of the mitotic spindles and the alteration of the hBUB1 gene has been found to be associated with chromosomal instability in some tumor cell lines. We analyzed the coding region of the hBUB1 gene for mutations and its expression in 92 acute myeloid leukemia (AML) specimens and five hematopoietic cell lines. We also used Southern hybridization to analyze the genomic DNA of those cases, which had aberrant transcription to confirm the lesion. A thymine/cytosine polymorphism at 8 bp upstream of the 5' splice acceptor site of exon 10 was observed in Raji cell line and two AML specimens without a resultant change in the expression of hBUB1. Reduced expression and aberrant transcription of the hBUB1 gene, which may affect the control of mitotic checkpoint, were detected in AML specimens by reverse transcription-polymerase chain reaction (RT-PCR) analysis. Our study suggests that the mutation of the hBUB1 gene is a rare event in AML, and further studies are necessary to clarify its role in leukemia.