As outlined above, some initiatives recruit individuals with a single strong risk factor such as carrier status for LRRK2 or GBA mutations, or idiopathic RBD or anosmia, in order that subjects may be followed prospectively, whereas other approaches employ large population-based cohorts or retrospective case–control methods to examine associations with PD and preceding diagnoses. This evidence concerns the gene LRRK2 and Parkinson disease.