In previous studies of mammary tumour origin, we used genetically engineered mouse models in which CRE recombinase-dependent conditional deletion of the tumour suppressor genes Brca1, Brca2, p53 (also known as Trp53) and/or Pten were targeted to basal or luminal ER− cells using the Krt14 or Blg promoters, respectively. The gene discussed is TP53; the disease is breast cancer.