Based on their sequence homology and domain organisation (Figure 1) [11,12], ABC transporters, such as the extensively studied multidrug resistance protein 1 (MDR1 a.k.a. P-glycoprotein, P-gp, and ABCB1), MDR-associated protein 1 (MRP1 a.k.a. ABCC1) and breast cancer resistance protein (BCRP a.k.a. ABCG2), enable cancer cells to elude “hazardous substances” by extruding them extracellularly through a process known as drug efflux [13]. This evidence concerns the gene ABCG2 and cancer.