We found that, among patients with PD, those with baseline pS129-α-synuclein levels > 8.46 fg/mL were at higher risk of motor symptom progression by at least 3 points in the MDS-UPDRS part III scores than those with pS129-α-synuclein levels < 8.46 fg/mL. The gene discussed is SNCA; the disease is Parkinson disease.