ABCG2 and cancer: The first eukaryotic ABC transporter discovered was the human (h) drug efflux transporter MDR (multidrug resistance) P-glycoprotein (hABCB1/MDR1), the name of which reflects that its expression in cancers can cause a decreased cellular drug accumulation (initially referred to as drug permeability, 'P'), resulting in the resistance of tumours against chemotherapy [14,15].