SCN9A and sodium channelopathy-related small fiber neuropathy: Loss-of-function mutation of the SCN9A gene that codes for Nav1.7 leads to congenital insensitivity to pain (Cox et al., 2006), whereas SCN9A gain-of-function mutations causes paroxysmal extreme pain disorder and primary erythromelalgia (Dib-Hajj et al., 2008).