The risk of developing PD follows the same “dose effect” principle: those with more severe GBA1 mutations (e.g., “neuronopathic” mutations) and/or biallelic carriage generally have lower GCase activity and are at higher risk of PD relative to patients with less severe mutations or mutations in only 1 chromosomal copy [22, 24–27]. The gene discussed is GBA1; the disease is Parkinson disease.