Of all the known genetic variants associated with Parkinson disease, mutations in GBA1, the gene encoding the lysosomal enzyme glucocerebrosidase (Glucosylceramidase Beta or GCase; EC 3.2.1.45), have a major advantage due to the association of this gene with a well-studied lysosomal storage disorder, Gaucher disease. Here, GBA1 is linked to Parkinson disease.