HEXA and Tay-Sachs disease: If both parents carry pathogenic variants of the HEXB gene, then each sibling of the affected child has a 25% chance of being affected with SD.[1] Once pathogenic variants of HEXB are identified in the affected family members, prenatal/preimplantation genetic testing can be performed for screening of SD in cases of pregnancies of high-risk relatives.[2] HEXA mutations cause Tay-Sachs disease (TSD).