SCN9A and hereditary sensory and autonomic neuropathy: Recent human association studies have directly linked SCN9A, the gene that encodes Nav1.7, to three human pain disorders: dominantly inherited gain-of-function mutations in inherited erythromelalgia (IEM; nine mutations), paroxysmal extreme pain disorder (PEPD; eight mutations), and recessively inherited loss-of-function mutations in Nav1.7-related congenital insensitivity to pain (CIP; fourteen mutations) (Dib-hajj et al., 2009).