Similarly, knock-in of the AKT1(E17K) mutation into MCF-7 ER-positive cells in which oncogenic PIK3CA(E545K) has been restored to the wild-type allele restores proliferation and tumor growth in vivo, arguing that at least in ER-positive luminal breast cancer cells, AKT1(E17K) can function as a bona fide oncogene [19]. This evidence concerns the gene PIK3CA and breast cancer.