In concert with published findings (Domchek et al., 2013; Freire et al., 2018; Sawyer et al., 2015; Seo et al., 2018), our data suggest that the phenotypic variability of biallelic BRCA1 mutations ranges from embryonic lethality to short body height, microcephaly, early onset BC and toxicity from chemotherapy, whereas residual BRCA1 protein function determines the disease severity. The gene discussed is BRCA1; the disease is microcephaly.