SCN9A and paroxysmal extreme pain disorder: In contrast, gain-of-function mutations of Nav1.7 are associated with inherited pain syndromes including inherited erythromelalgia (IEM) (Yang et al., 2004, 2016; Waxman, 2013), paroxysmal extreme pain disorder (PEPD) (Fertleman et al., 2006; Jarecki et al., 2008), and idiopathic small fiber neuropathy (SFN) (Faber et al., 2012).