The demonstration that Nav1.4 LoF causes loss of muscle force has been provided in the last 5 years with the identification and the functional investigation of recessively-inherited SCN4A mutations in children with two forms of congenital muscle weakness: congenital myasthenic syndrome (CMS) and congenital myopathy (CM). The gene discussed is SCN4A; the disease is congenital myopathy with cores.