SCN9A and hereditary sensory and autonomic neuropathy: Clinical genetic studies from several groups have shown that loss of function of the Nav1.7 (SCN9A) voltage-gated sodium ion channel leads to complete inability to perceive pain (congenital indifference to pain or CIP) [1]–[3], whereas Nav1.7 gain of function alleles cause or contribute to chronic spontaneous pain [4]–[7].