ABCG2 and breast cancer: Our recent studies have focused on ABCG2 (BCRP), first identified due to its overexpression in multidrug-resistant breast cancer cell lines and later found to be highly expressed in a variety of human cancers including acute myelogenous leukemia, acute lymphoblastic leukemia, breast and lung cancers (1), where its overexpression has been correlated with poor prognosis.