The finding of high numbers of GFP+ cells in the pubertal mammary stem cell population, although not fully interrogated for HDR contribution by site loss determination, suggests that the genomic instability incurred in these cells as a result of BRCA2 deficiency would be propagated to both luminal and basal daughter cells, which would also contribute to tumour heterogeneity. Here, BRCA2 is linked to neoplasm.