BRCA1 dysfunction is a well-established cause of homologous recombination deficiency (HRD), yet cells with concurrent loss of both BRCA1 and 53BP1 can paradoxically regain HR proficiency (Bouwman et al., 2010; Jaspers et al., 2013). The gene discussed is TP53BP1; the disease is hypoparathyroidism-retardation-dysmorphism syndrome.