We show that induced loss of p53 in luminal MECs leads to their clonal expansion without directly affecting their luminal fate, but predisposes luminal cells to the development of mammary tumours with loss of luminal identity and acquisition of MaSC-like properties.Thus, our data suggest that although p53 does not dictate the identity of luminal MECs directly, it safeguards them from aberrant proliferation, cell fate alteration, and development of mammary tumours with loss of their original luminal MEC properties. The gene discussed is TP53; the disease is breast cancer.