We have previously demonstrated that mouse mammary tumours developing in Brca1 p53, Brca2 p53 or Pten p53 loss-of-function models, or in a Her2/Neu gain-of-function model, can be grouped into one of four distinct histotypes: adenomyoepithelioma (AME), metaplastic adenosquamous carcinoma (ASQC), metaplastic spindle cell carcinoma (MSCC) and adenocarcinoma of no special type (AC(NST)) [1,2,3]. The gene discussed is ERBB2; the disease is breast cancer.