Gain-of-function mutations in NaV1.7 cause human pain disorders, such as inherited erythromelalgia (IEM) [16, 18] and small fiber neuropathy (SFN) [10], whereas loss-of-function mutations of NaV1.7 lead to congenital insensitivity to pain (CIP) [7, 13]. Here, SCN9A is linked to erythromelalgia.