Thus carcinogenesis in BRCA1/2 germline mutation carriers may take place in two hypothetical steps: the documented haploinsufficiency accelerates the inactivation of the second BRCA1/2 allele via large-scale rearrangements, and the high SNV mutagenesis rate in the arising BRCA1/2 homozygote leads to faster mutation of further cancer genes. The gene discussed is BRCA1; the disease is cancer.