BRCA1 and breast carcinoma: The fact that, when exposed to sufficient levels of replication stress, conditional BRCA1 HR-DSBR haploinsufficiency emerged in BRCA1 heterozygous cells suggests that this defect might join SFR haploinsufficiency as a BRCA1 breast cancer co-contributor in mammary epithelial progenitor cells that are experiencing sufficient ongoing replication stress.