Our main findings were that in PD patients with advanced disease (1) serum NfL levels were increased in PIGD subtype (2) PIGD patients had worse MoCA, but similar MDS-UPDRS III total scores compared with non-PIGD patients and (3) increased serum NfL concentrations were incrementally associated with higher PIGD score. The gene discussed is NEFL; the disease is Parkinson disease.