SCN9A and erythromelalgia: Gain-of-function variants of Nav1.7 are linked to inherited pain syndromes such as erythromelalgia (Yang et al., 2004; Lampert et al., 2010; Bennett and Woods, 2014; Brouwer et al., 2014; Tang et al., 2015; Dib-Hajj et al., 2017) or paroxysmal extreme pain disorder (Fertleman et al., 2006, 2007; Jarecki et al., 2008; Stępień et al., 2020).