The most frequent form of monogenic PD is caused by autosomal dominant mutations in LRRK2, encoding leucine-rich repeat kinase-222,23, of which the G2019S mutation is the most common.24 The normal function of the lrrk2 protein is incompletely understood, but current evidence indicates a role in pathways relating to vesicular transport and lysosomal degradation. This evidence concerns the gene LRRK2 and Parkinson disease.