F8A1 and Huntington disease: Notably, among human HD patients, the rare HD homozygosity does not apparently lower the age of disease onset than the typical heterozygotes [65–67], implying a dosage threshold and potentially a saturating effect by mutant HTT in inflicting neuronal damage, which would discount against the postulated pathogenic role of HAP40 through its impact on the protein levels of mutant HTT.