SCN9A and erythromelalgia: In particular, loss-of-function mutations in SCN9A (the gene encoding Nav1.7) have been identified in patients with congenital insensitivity to pain (CIP; [3]), whereas gain-of-function mutations in SCN9A lead to chronic pain syndromes such as paroxysmal extreme pain disorder (PEPD, [4]) and inherited erythromelalgia (IEM) [5] [6] [7] [8].