The first gene to be linked to PD was SNCA, which encodes α-synuclein (α-syn).1 Interestingly, both mutations in the SNCA gene and increased copy number of wild-type α-syn lead to PD.2 Moreover, α-syn aggregates to form Lewy bodies.3 The most commonly mutated gene among PD patients is LRRK2, which encodes a Leucine Rich Repeat Kinase.4,5 Mutations in SNCA and LRRK2 give rise to an autosomal dominant form of PD and are thought to result from a toxic gain of function. This evidence concerns the gene LRRK2 and Parkinson disease.