Congenital insensitivity to pain is due to loss-of-function mutations in NaV1.7 and results in loss of pain [12, 13], whereas primary erythromelalgia, paroxysmal extreme pain disorder, and small fiber neuropathy are due to gain-of-function mutations in NaV1.7 and result in ongoing spontaneous pain [14–16]. The gene discussed is SCN9A; the disease is primary erythermalgia.