To directly demonstrate that in virgin mice high levels of endogenous pSTAT5 can mimic pregnancy in increasing breast cancer risk, we took advantage of our newly characterized transgenic WAP-tva mouse line (Haricharan et al., 2013), which expresses tva from the promoter of the Wap (whey acidic protein) gene—a classical transcriptional target of STAT5 (Hennighausen and Robinson, 2008)—and used it for selective delivery of caErbb2 into the pSTAT5+ subset of mammary epithelial cells in virgin mice. Here, STAT5B is linked to breast carcinoma.