AR and X-linked disease: SBMA is a neuromuscular disease caused by a toxic gain of function mechanism, in which both muscle and motor neurons are affected.8, 9 SBMA (Kennedy's disease; OMIM #313200) is an X-linked disease, characterized by muscle weakness and atrophy, caused by CAG repeat expansion in the AR gene, which encodes an extended polyglutamine tract in the AR protein.10 No disease-modifying treatment is currently available for this disorder.