The retinoblastoma tumor suppressor, Rb, is implicated in luminal-B and basal-like breast carcinomas, yet its effect on mammary gland development and causal role in breast cancer subtypes remain undefined. Here we show that conditional deletion of Rb in mouse mammary epithelium led to expansion of the stem/progenitor cells and to focal acinar hyperplasia with squamous metaplasia. These uniform lesions progressed into histologically diverse, transplantable mammary adenocarcinomas and adenosquamous carcinomas with features of luminal-B or basal-like carcinomas. A subset of basal-like but none of the luminal-B tumors expressed mutant p53. These results demonstrate a causative role for Rb in the etiology of breast cancer subtypes and implicate p53 status as a determinant of tumor phenotype after Rb loss.
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