Despite having only sequenced a portion of the tumors with BRCA1/2 variants in our cohort (30/77), a mono-allelic loss-of-function mutation in BRCA1 and BRCA2 was more frequent than within the control cohort4: ≥2/1995 (0.1%) vs. 5/11,241 (0.04%) for BRCA1 and ≥8/1995 (0.4%) vs. 15/11,241 (0.13%) for BRCA2. Thus, mono-allelic mutation alone does seem to play a role in the development of breast cancer, which is supported by several biological studies showing the effects of haploinsufficiency of BRCA1/2 in carcinogenesis26,27. This evidence concerns the gene BRCA2 and breast cancer.