Furthermore, the whole idea of selective chemosensitivity of BRCA-related BC is based on the “two-hit” hypothesis; however, some of the recent data indicate, that BCs arising in BRCA1 mutation carriers do not necessarily display the loss of the remaining allele, and haploinsufficiency of heterozygous BRCA1 cells may at least in some instances contribute to tumorigenesis [14-16]. This evidence concerns the gene BRCA1 and breast cancer.