SCN9A and primary erythermalgia: Among the four genes, SCN9A-mapped mutations in pain disorders have been studied fully—e.g., p.Arg 222 His, evaluated from patients from a large family with early-onset pain symptoms, a familial mutation (I136V) and two sporadic mutations (I848T, V1316A) identified in patients with primary erythromelalgia (PE) [66,67,68].