However, ixabepilone is not a substrate of the breast cancer resistance protein (BCRP1/ABCC-2), a protein that is significantly overexpressed in doxorubicin- and paclitaxel-resistant breast cancer cells (MCF-7/DOX and MCF-7AX), which explains the potency of ixabepilone- in taxane-resistant breast cancer [66]. This evidence concerns the gene ABCG2 and breast cancer.