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. The gene discussed is SLC6A5; the disease is hyperekplexia.