The ABC transporter ABCG2, also known as breast cancer resistance protein (BCRP) or mitoxantrone resistance-associated protein (MXR), was cloned independently in 1998 from both a drug-selected human breast cancer cell line and a human cDNA library (Allikmets et al., 1998; Doyle et al., 1998). Here, ABCG2 is linked to breast cancer.