['Aberrant DNA methylation status of some genes has been shown to be involved in chemoresistance of acute myeloid leukemia (AML). We have recently found that down-regulation of the β subunit of mitochondrial ATP synthase (ATPsyn-β) leads to adriamycin resistance in acute and chronic myeloid leukemia cells, and hypermethylation of the ATPsyn-β gene promoter is associated with chemoresistance in chronic myeloid leukemia.', 'To further investigate the relationship between methylation of ATPsyn-β gene, mRNA expression as well as chemoresistance in AML.', 'Quantitative RT-PCR and methylation specific PCR were performed to assess mRNA expression and methylation status of ATPsyn-β gene on primary bone marrow nuclear cells (BMMCs), and cell proliferation assay was used to determine the sensitivity of BMMCs to adriamycin.', 'Hypermethylation status of ATPsyn-β gene promoter existed in those relapsed/refractory AML patients, and this hypermethylation of the gene was associated with a suppressed mRNA expression levels. Four patients at diagnosis and relapse underwent gene methylation status shift from hypermethylation to hypomethylation, which was accompanied by reduced mRNA expression of the gene. 5-azacitidine(5-Aza)- a demethylating agent, could restore ATPsyn-β mRNA expression and increase the adriamycin sensitivity of primary leukemic cells from seven relapsed/ refractory AML patients.'] Is hypermethylation of CpG sites at the promoter region associated with deregulation of mitochondrial ATPsyn-β and chemoresistance in acute myeloid leukemia?