Although epirubicin is not considered a substrate for BCRP, a recent study has shown that high expression of BCRP correlated with resistance to epirubicin in colorectal cancer cells [42] and this is consistent with our findings in the SN-38 resistant MDA-MB-231 cell lines with high BCRP expression. The gene discussed is ABCG2; the disease is colorectal cancer.