iPS-PyVT cells probably first differentiate into mammary stem cells under the paracrine influence of mammary fat pad microenvironment and then mature into breast ducts that then transform under the influence of the oncogenic transgene into precancerous and then invasive breast cancer, mirroring the same sequence observed in the FVB/N-Tg (MMTV-PyVT)634Mul/J and FVB-Tg (MMTV-ErbB2) NK1Mul/J transgenics. The gene discussed is ERBB2; the disease is invasive breast carcinoma.