The observations of Drost and co-workers support this hypothesis, who reported that mice carrying the pathogenic p.Cys61Gly variant, inactivating BRCA1 E3 ligase activity [37], are embryonically lethal because of a developmental delay and that, in addition, the latency and nature of mammary tumors with one Brca1 allele carrying the p.Cys61Gly variant and one Brca1 conditional null allele were similar to those carrying two Brca1 null alleles [63]. Here, BRCA1 is linked to breast cancer.