Asymptomatic carriers of LRRK2 mutations constitute an ideal population for identifying predictive biomarkers of PD for several reasons: 1) a high risk of conversion to PD, 2) dopaminergic neuronal loss demonstrated by positron emission tomography (PET) scanning, and 3) similarity of the clinical phenotype of LRRK2-associated PD to that of patients with sporadic PD (sPD). The gene discussed is LRRK2; the disease is Parkinson disease.