Following the identification of ABCB1 [1,2] and ABCC1 [3], ABCG2 (also known as BCRP/MXR) was discovered in mitoxantrone-resistant human colorectal cancer cells and anthracycline-resistant breast cancer cells [4,5,6], and has since been shown to mediate the efflux of a broad spectrum of anticancer agents [7]. This evidence concerns the gene ABCG2 and breast cancer.