ABCG2 is a member of the ATP-binding cassette (ABC) transporter superfamily and over-expression of ABCG2 has been shown to cause multidrug resistance (MDR) in model cancer cell lines and to correlate with poor prognosis in both adult and childhood leukemia and breast cancer patients (for reviews see [1], [2], [3]). Here, ABCG2 is linked to breast cancer.