Since the requirement for BRCA2 for efficient HDR is thought to be central to its tumour suppressor role, our observation that Brca2Δ27 mutation reduced HDR similarly in each of the mammary epithelial cell subpopulations analysed suggests that multiple possible cells-of-origin for mammary tumours are at risk with BRCA2 mutation, consistent with the heterogeneous nature of BRCA2-associated breast cancers. This evidence concerns the gene BRCA2 and breast carcinoma.