Among the ABC proteins, the most significant are glycoprotein P (P-gp), encoded by the ABCB1 [multidrug resistance protein 1 (MDR1)] gene, and breast cancer resistance protein (BCRP or ABCG2), which was cloned from a mitoxantrone-resistant subline of the breast cancer cell line MCF-7. The gene discussed is ABCG2; the disease is breast cancer.