However, it is important that myotonia is not erroneously attributed to an identified CLCN1 variant as myotonia can also be caused by myotonic dystrophy (DM), a multisystem and potentially lethal disorder, and by gain-of-function mutations of the skeletal muscle sodium channel Nav1.4 (encoded by SCN4A). The gene discussed is CLCN1; the disease is Myotonia.