This review aimed to describe the clinical differences among patients with various pathogenic genes associated with PD or Parkinsonism to highlight potential underlying mechanisms regulating these genes, with a particular focus on SNCA, LRRK2, VPS13C, glucosylceramidase beta (GBA1), GCH1, and microtubule-associated protein tau (MAPT). The gene discussed is GCH1; the disease is Parkinson disease.