SCN9A and paroxysmal extreme pain disorder: Conversely, gain of function of Nav1.7 leads to a triad of autosomal dominant familial pain syndromes, including inherited erythromelalgia (IEM), paroxysmal extreme pain disorder (PEPD; prevalence <1/1000,000), and painful small-fiber neuropathy (SFN; Yang et al., 2004; Fertleman et al., 2006; Faber et al., 2012a).