SCN9A and erythromelalgia: Gain-of-function mutations in SCN9A (the gene encoding NaV1.7) lead to inherited erythromelalgia (IEM; pain and erythema of the hands and feet) (Yang et al., 2004) or paroxysmal extreme pain disorder (PEPD; paroxysmal pain and erythema of the sacrum and mandible) (Fertleman et al., 2006), while loss-of-function mutations lead to pain insensitivity (Cox et al., 2006).