Among various ABC transporters, the main ABC transporters primarily linked to MDR in breast cancer are P-glycoprotein (P-gp/MDR1/ABCB1), multidrug resistance-associated protein 1 (MRP1/ABCC1), and breast cancer resistance protein (BCRP/ABCG2) [20,22,24]. The gene discussed is PGP; the disease is breast cancer.