Upregulation of BCR-ABL1 protein due to amplification of the BCR-ABL1 fusion gene18 and overexpression of P-gp19 were observed in the imatinib-resistant sublines established after long-term culture of imatinib-sensitive Ph+ leukemia cell lines in the presence of increasing concentrations of imatinib. This evidence concerns the gene ABL1 and leukemia.