1999) but have yielded important pathophysiological insights through the identification of novel genes contributing to PD susceptibility. Mutations in five genes result in Mendelian forms of PD. Two genes, leucine-rich repeat kinase 2 (LRRK2) and synuclein, alpha (non-A4 component of amyloid precursor) (SNCA), are mutated in autosomal dominant forms of PD. Three genes, parkin RBR E3 ubiquitin protein ligase (PARK2), DJ1 (PARK7) and PTEN-induced putative kinase 1 (PINK1) are mutated in autosomal recessive forms of PD. Here, LRRK2 is linked to Parkinson disease.