In MDR sublines of the small cell lung cancer cell line GLC-4, the non-small cell lung cancer cell line SW1573, the epidermal cervical cancer cell line KB-3-1 and the promyelocytic leukemia cell line HL-60, the promoter methylation patterns of the three ABC transporters were determined in order to contribute to elucidation of the role of DNA methylation changes in acquisition of a MDR phenotype. The gene discussed is ABCG2; the disease is acute promyelocytic leukemia.