SCN9A and paroxysmal extreme pain disorder: Gain-of-function mutations of Nav1.7 are implicated in two distinct paroxysmal pain syndromes—IEM (primary erythermalgia) and PEPD (paroxysmal extreme pain disorder), while loss-of-function mutations of Nav1.7 inflict people with CIP (indifference to pain, congenital, autosomal recessive) (Lampert et al., 2010; Dib-Hajj et al., 2013).