The myelodysplastic syndromes (MDS) comprise a group of clonal hemopoietic stem cell disorders characterized by ineffective hematopoiesis with an increased propensity to myeloid leukemic (AML) transformation. The underlying molecular basis for MDS and its leukemic evolution is unclear. Except for patients with 17p syndrome, loss of function of the p53 tumor suppressor gene accounts for <10% of MDS and AML cases. Recently, mutations of the checkpoint gene, CHK2, the human homologue of the yeast CDS1 and RAD53 genes, have been reported in patients with Li-Fraumeni syndrome who also have normal p53. As p53 mutations are rare in MDS and AML, we investigated the status of the CHK2 gene by reverse transcriptase-polymerase chain reaction (RT-PCR) in patients with MDS (n=10) and patients in whom MDS had transformed into AML (n=3). In the MDS group, we found one patient with a conserved mutation (Lys-->Arg) in the forked head-associated (FHA) domain of the CHK2 coding sequence. We also found a deletion in the CHK2 transcript in one patient from the MDS-->AML group, resulting in a truncated protein lacking the kinase domain. We conclude that alterations of CHK2 and possible involvement in the pathogenesis of MDS may be a rare event.