Voltage-gated sodium channels are selective ion channels that regulate the permeability of sodium ions in excitable cells. During the propagation of an action potential, sodium channels allow an influx of sodium ions, which rapidly depolarizes the cell. Na+ CP type IXalpha, also known as SCN9A (sodium channel protein type 9 subunit alpha), NENA, PN1, Nav1.7 or ETHA, is a 1,988 amino acid multi-pass membrane protein that belongs to the voltage-gated sodium channel family. Expressed in dorsal root ganglion, smooth muscle cells and in the central and peripheral nervous system, Na+ CP type IXalpha functions to mediate the voltage-dependent sodium ion permeability of membranes, specifically forming a sodium-selective ion channel through which sodium may pass. Via its ability to control the flow of sodium in and out of excitable membranes, Na+ CP type IXalpha plays an important role in the inflammatory pain response. Defects in the gene encoding Na+ CP type IXalpha are the cause of primary erythermalgia, autosomal recessive congenital indifference to pain and paroxysmal extreme pain disorder (PEPD), all of which are genetic pain disorders.