ABCG2 and cancer: Different strategies to overcome MDR in cancer have been proposed, albeit it remains an important oncological challenge, since several cellular mechanisms are involved in MDR, including the inactivation of the drug, enhanced DNA repair, mutations or altered expression levels of the biological target, drug compartmentalization, altered mitochondria, failure of programmed cell death and overexpression of ABC transporters (Gottesman et al., 2002[17]; Hall et al., 2009[22]; Szakács et al., 2014[53]).