SCN4A and cutaneous mastocytosis: Dominant gain of function mutations in the α-subunit of the skeletal muscle voltage-gated sodium channel (Nav1.4) gene (SCN4A) are associated with a variety of muscle phenotypes, including potassium-aggravated myotonia, hypokalaemic periodic paralysis, paramyotonia congenita and hyperkalaemic periodic paralysis, while recessive loss of function mutations are linked to congenital myasthenic syndrome and classical CM [215].