The discovery of glucocerebrosidase (GBA) mutations in PD originates from the observation of increased incidence of parkinsonism in patients and obligate mutation carriers in families of Gaucher disease (GD), an autosomal recessive lysosomal storage disease caused by homozygous and compound heterozygous GBA mutations which lead to reduced glucocerebrosidase (GCase) enzymatic activity [75, 76]. This evidence concerns the gene GBA1 and Parkinson disease.