Inherited erythromelalgia (IE)43 and paroxysmal extreme pain disorder (PEPD)44 are caused by mutations of the SCN9A gene that encodes the sodium channel Nav 1.7.3 IE is characterised by episodes of symmetrical burning pain of the feet or legs, often accompanied by reddening of the extremities (figure 6). This evidence concerns the gene SCN9A and erythromelalgia.