SCN9A and hereditary sensory and autonomic neuropathy: Mutations in the SCN9A gene can be categorized as gain-of-function mutations (increased Nav1.7 activity), causing paroxysmal extreme pain disorder (PEPD) and erythromelalgia (EM); or loss-of-function mutations (reduced Nav1.7 activity), causing congenital insensitivity to pain (CIP) [18].