ABCG2 and cancer: As a member of the ABC family, overexpression of breast cancer resistance protein (BCRP/ABCG2) may be observed in normal tissues (like the placenta, intestine, liver, blood-testis or brain barrier, hematopoietic progenitor and other stem cells) and chemoresistant cancer cells for the efflux of cytotoxic drugs such as mitoxantrone.