ABCB1 and neoplasm: Resistance to different chemotherapeutic regimen involves intrinsic and extrinsic mechanisms and those are determined by pre-existing genetic and/or epigenetic properties of malignant cells including enhanced drug efflux pumps (i.e. P-glycoprotein; MDR1; MRP1), blunted apoptotic signaling, increased metabolic activities, loss of specific oncogenes, gain of stem cell plasticity, strengthen DNA damage repair machinery, tumor heterogeneity within tumor microenvironment (TME) (11, 12).