We hypothesised that as LYN kinase activity was required for survival of cells that had lost BRCA1 activity (Tornillo et al., 2018), introducing the conditional Lynfl(ex4) allele into the BlgCre Brca1fl/fl p53+/− background would result in a significant increase in overall survival (i.e. the age at which mice had to be euthanised for any reason) and also in mammary tumour-specific survival (i.e. the age at which mice had to be euthanised specifically as a result of the size of a mammary tumour). This evidence concerns the gene BRCA1 and breast cancer.