SCN9A and neuropathy, small fiber: Mutations that inactivate the SCN9A gene result in both congenital insensitivity to pain and hereditary sensory and autonomic neuropathy type IID.25 Gain-of-function mutations in this gene produce syndromes such as inherited erythromelalgia, paroxysmal extreme pain disorder (familial rectal pain), and small-fiber neuropathy.25 Other voltage-gated sodium channel subtypes have also been associated with pain disorders, including Nav1.8 (SCN10A) with small-fiber neuropathy and Nav1.9 (SCN11A) with congenital insensitivity to pain.25