Gain-of-function mutations of the NaV1.7 sodium channel, which is preferentially expressed at relatively high levels within peripheral (dorsal root ganglion and sympathetic ganglion) neurons [1-3] produce several syndromes associated with severe pain, including inherited erythromelalgia [4-8] and paroxysmal extreme pain disorder [9,10] as well as painful small-fiber neuropathy [11,12], while loss-of-function mutations of NaV1.7 cause channelopathy-associated insensitivity to pain [13-15]. The gene discussed is SCN9A; the disease is erythromelalgia.