In addition to T1 and T7, we found three other spontaneous tumors (named here T9, T10 and T11) with more than 10% ABCG2-positive tumors cells in our panel of 28 Brca1−/−;p53−/− mammary tumors derived from FVB/N mice (Zander et al., submitted for publication). Here, BRCA1 is linked to breast cancer.