TP53 and neoplasm: Since all tumours developed in these animals have a luminal origin and induced loss of p53 in luminal MECs does not lead to loss of the luminal cell fate directly, we reasoned that other oncogenic events, such as those caused by genomic abnormalities triggered by p53 loss, might drive Trp53-null luminal MECs toward a basal or mesenchymal-like cell fate.