Although the penetrance of D409H GBA1 mutation is relatively lower than other mutations such as N370S and L444P GBA1 mutations [40], our current findings suggest that GBA1 deficiency due to D409H GBA1 mutation alone is not sufficient to cause PD but additional factors, such as environmental factors or increased levels of α-synuclein could increase the penetrance through rendering the levels of α-synuclein accumulation close to the threshold required for α-synuclein aggregation. The gene discussed is GBA1; the disease is Parkinson disease.