ABCB1 and chronic myelogenous leukemia, BCR-ABL1 positive: The success of the first-line therapy option BCR-ABL inhibitor imatinib has been opposed with drug resistance that occurs due to both intrinsic and extrinsic factors including ABL-kinase region mutations, BCR-ABL gene amplification/overexpression, clonal evolution, the presence of CML stem cells, the overexpression of MDR1 (multidrug resistance protein 1), and decreased drug bioavailability (Apperley 2007).