SCN9A and paroxysmal extreme pain disorder: The Nav1.7 sodium channel is preferentially expressed in sensory and sympathetic neurons and has been directly linked to painful disorders in humans; genetic studies have identified gain-of-function missense mutations within SCN9A, the sodium channel gene that encodes Nav1.7, in patients with inherited erythromelalgia (IEM), and a different set of gain-of-function missense mutations has been found in patients with paroxysmal extreme pain disorder (PEPD) [2,3].