SCN9A and hereditary sensory and autonomic neuropathy: Furthermore, gain-of-function mutations in the Scn9A gene were found to be linked with paroxysmal extreme pain disorder (PEPD) [58] and idiopathic small fiber neuropathy [59]; whereas, loss-of-function NaV1.7 mutations produce congenital insensitivity (or indifference) to pain (CIP), which is a disease in which patients experience painless fractures, lacerations, burns, and tooth extractions, for example [60].