SCN4A and Myotonia: Functional studies have confirmed a clear deleterious impact of this mutation on channel function, with impaired fast inactivation and accelerated recovery from inactivation.13 A gain‐of‐function effect mediated via impaired fast inactivation is the hallmark of all SCN4A myotonia‐causing mutations.1 The p.A1156T mutation is present at low frequency (15 individuals) in the gnomAD database.