SNCA and Parkinson disease: Early candidate gene studies have revealed that only a small percentage of the sporadic PD cases carry mutations in a number of previously known Mendelian PD genes including SNCA, PARKIN, LRRK2, and GBA1 (Table 1) (Maraganore et al. 2006; Satake et al. 2009; Zabetian et al. 2009).