ABCG2 and breast carcinoma: Secondly, since overexpression of the ABC transporter pump is one of the common mechanisms mediating multi-drug resistance in tumors, we examined the expression of P-gp, which plays a major role in breast cancer together with BCRP, the most well-characterized drug transporter in the ABC protein superfamily, encoded by the ABCB1 (formerly known as MDR1).