In this study deletion of Brca1 in Blg (β-lactoglobulin)-positive luminal progenitors from p53+/− mice produced ER-basal tumors that phenocopied Brca1 mutant tumors, whereas depletion of Brca1 in the CK14+ basal progenitor produced basal-like breast cancers that did not resemble those found in human patients, suggesting that Brca1 tumors likely arise from a luminal progenitor. The gene discussed is BRCA1; the disease is breast cancer.