SCN4A and Fatigable weakness: However, two-fold reduction in skeletal muscle Nav1.4 current in Scn4a haploinsufficient mice (i.e. heterozygous for a null Scn4a allele) induced muscle weakness in response to repetitive stimulations but not in response to PP triggering factors (K+, glucose and insulin challenges), thereby clinically mimicking myasthenia i.e. fatigable muscle weakness in response to repetitive stimulation rather than PP28.