Among the mechanisms that mediate drug resistance, the most widely studied mechanism is drug efflux through the membrane, given that tumor cells overexpress drug transport proteins located in the cytoplasmic membrane, including the ATP-binding cassette (ABC) transporters P-gp (P-glycoprotein, ABCB1), multidrug resistance-associated protein 1, ABCC1, and breast cancer resistance protein, ABCG2 (BCRP) [6,7]. The gene discussed is ABCG2; the disease is neoplasm.