SCN4A and Fatigable weakness: All Nav1.4 channelopathies have an autosomal dominant mode of inheritance except those with primary congenital muscle weakness (myasthenia, myopathy and hypokinesia), which result from recessively-inherited loss-of-function mutations of SCN4A. Dominant SCN4A mutations resulting in hyperPP usually impair the inactivation properties of Nav1.44,6.