Facioscapulohumeral muscular dystrophy (FSHD) is a common inherited muscle disorder (prevalence 5–12/100 000) caused by epigenetic derepression of the normally silenced DUX4 gene.1 It produces progressive weakness of facial, shoulder girdle and later limb muscles, with no current targeted therapy. Myasthenia gravis (MG), in contrast, is an autoimmune neuromuscular junction disorder most often due to antiacetylcholine receptor (AChR) antibodies, causing fatigable weakness that typically affects extraocular, bulbar, respiratory and proximal limb muscles. MG is treatable but requires timely recognition to prevent life-threatening crises and has a UK prevalence of 10–20/100 000.2 Although both conditions are rare, several reports describe their co-occurrence. FSHD is associated with inflammatory changes in skeletal muscle, and DUX4 is expressed in the thymus, an organ central to T cell tolerance.3 This pilot study, therefore, aimed to determine whether MG occurs more frequently in FSHD and to explore whether FSHD may…