To test our hypothesis that decreased GBA1 enzyme activity due to mutation in GBA1 affects neurodegeneration in the hA53T α-synuclein transgenic (Tg) mouse model of PD, the GBA1D409H/D409H mutant mice [45] were crossbred with the hA53T α-synuclein (α-Syn) Tg mice (Fig. 1). This evidence concerns the gene GBA1 and Parkinson disease.