In cancer cells, three ABC transporters are considered most relevant, with undoubted association with chemotherapeutic treatment failure: P-glycoprotein (P-gp or MDR1, encoded by ABCB1), multidrug resistance-associated protein 1 (MRP1, encoded by ABCC1) and breast cancer resistance protein (BCRP, encoded by ABCG2) (Gottesman et al., 2002[17]; Szakács et al., 2006[54]). Here, ABCG2 is linked to cancer.