LRRK2 and Parkinson disease: Early-onset PD, in which subjects present with PD between the ages of 21 and 50, is often associated with familial inheritance caused by gene mutations in 18 specific chromosomal regions/PD-related loci (PARK1–18), such as the SNCA gene (α-synuclein; PARK1 and PARK4), Parkin (ubiquitin protein ligase; PARK2), DJ-1 (PARK7), or LRRK2 (PARK8) [2,3] (Table 1).