GBA1 and Parkinson disease: Although the mechanisms by which GBA1 mutations cause PD are not fully understood yet, the following evidence suggests that a reduced GBA1 enzymatic activity is responsible for the increased risk of developing PD: (1) There is a negative correlation between GBA1 activity and asyn accumulation [17, 48]; (2) heterozygous GBA1 mutations with lower enzymatic activities confer a higher risk of developing PD than those with higher enzymatic activities [49]; (3) GBA1 overexpression ameliorates asyn accumulation in a PD mouse model [50].