SCN4A and Fatigable weakness: To date, >40 dominant mutations in SCN4A have been reported to cause variable phenotypes, depending on the type and location of the mutations.1,2 In addition, rare recessive SCN4A mutations have been associated with congenital myasthenia or congenital myopathy.3,4 The type of channel defect and the degree of depolarization account for clinical symptoms; membrane hyperexcitability causes myotonia, increased membrane depolarization, and inexcitability, leading to paralysis,5, –, 7 while membrane hypoexcitability underlies myasthenias and myopathies.