BRCA1 and breast cancer: This approach somatically inactivates Brca1 and Trp53 in luminal mammary epithelial cells, including luminal progenitors (LPs), which are believed to serve as the cellular origin of BRCA1-associated basal-like breast cancer [40–42]; we showed by single-cell analysis in this model that LPs were the target luminal mammary epithelial subset that underwent expansion, eventually leading to development of mammary tumors that closely resemble the human basal-like breast cancer subtype [38].