Homologous recombination deficiency (HRD) associated with inactivation of mainly BRCA1 or BRCA2 (BRCA1/2) genes plays a major role in tumorigenesis of breast and ovarian cancers, has diagnostic and prognostic value for clinical management of the patients and recently emerged as a biomarker for the treatment targeting repair pathways. The gene discussed is BRCA1; the disease is hypoparathyroidism-retardation-dysmorphism syndrome.