Overexpression of Multidrug-Resistance Protein 1 (MDR1)/P-glycoprotein (PgP, encoded by Abcb1a/b) has been observed in BRCA-mutated breast and ovarian cancer cells, leading to a reduced intracellular concentration of PARP inhibitors, and consequently, PARP inhibitor resistance [137,138]. The gene discussed is ABCB1; the disease is ovarian carcinoma.