BRCA1 and neoplasm: For example, homozygous Brca1 and Brca2 knockouts die early in embryogenesis, and heterozygous mice are not tumour-prone, but mice harbouring a Cre-mediated deletion of Brca1[160] or Brca2 and Trp53[161] in the adult mammary gland do develop mammary tumours.