In a mouse model of mammary cancer, which normally develops four different histological tumour types upon knockout of the Pten and p53 tumour suppressor genes in the mammary gland, the additional knockout of the Notch1 or Notch2 genes did not alter the kinetics of tumour onset but did significantly change the relative proportions of different tumour types. The gene discussed is NOTCH1; the disease is neoplasm.