Intracellular recording in both ClCadr muscle and a pharmacologic model of Becker disease (due to block of ClC-1 with 9-AC) have elucidated a novel phenomenon: transient depolarizations to voltages between −25 and −35 mV, lasting many seconds, which we termed ‘plateau potentials.’ Blocking Na+ persistent inward current (NaPIC) with ranolazine prevented both development of plateau potentials and transient weakness. Here, CLCN1 is linked to myotonia congenita, autosomal recessive.