Nav1.7 is expressed predominantly in nociceptive and sympathetic neurons. Gain-of-function mutations have been described in Nav1.7 that result in extreme pain disorders such as SCN9A-associated small fiber neuropathy. In the disease states genetically linked to a gain-of-function of Nav1.7, the sodium channel is mutated to increase the sodium influx resulting in a hyperexcitable sensory neuron, and a resultant sensation of pain.