ABCG2 and breast cancer: In order to assess whether the inhibitors were specific for P-gp or would also inhibit other ABC transporters, we created a BCRP-overexpressing breast cancer cell line, MCF-7 M100, which we derived from MCF-7 cells48 by exposing the cells to increasing, sub-lethal concentrations of the BCRP pump substrate and chemotherapeutic, mitoxantrone49.