The most severe clinical manifestations result from mutations in SLC6A3, SLC6A5, SLCA18, and SLCA19 giving rise to infantile dystonia and parkinsonism (5, –, 7), hyperekplexia (9, –, 11), and Hartnup disease (45, –, 47), respectively. This evidence concerns the gene SLC6A5 and Parkinson disease.