A genetically engineered mouse model about BRCA1-associated breast cancer (Brca1−/−p53−/− mice) has identified that ABCG2 overexpression is the cause of acquired topotecan resistance, and the genetic ablation of ABCG2 improves the survival rate of topotecan-treated animals (Zander et al., 2010). This evidence concerns the gene ABCG2 and breast cancer.