The at-risk group includes 74 patients with idiopathic rapid eye movement (REM) sleep behaviour disorder (RBD), diagnosed with polysomnography according to International Classification of Sleep Disorders criteria12 (for more details see Barber et al4) patients with RBD with a pathogenic mutation of the GBA gene (RBD-GBA, n=3), and asymptomatic carriers of GBA gene pathogenic mutations (aGBA, n=8). The gene discussed is GBA1; the disease is sleep disorder.