Voltage-gated Na+ channels regulate the permeability of excitable cells to sodium ions. During the propagation of an action potential, Na+ channels allow an influx of sodium ions, which rapidly depolarize the cell. The sodium channel protein is comprised of one (alpha) subunit and two (beta) subunits. The Na+ CP type I and Na+ CP type II(alpha) subunits are expressed in adult brain. Na+ CP type III(alpha) is expressed in embryonic brain, but not in adult brain. Na+ CP type III(beta) is a 215 amino acid, single-pass type I membrane protein that modulates sodium channel gating kinetics and inactivates the channel opening more slowly than the I(beta) subunit. It has an extracellular N-terminal domain, an N-terminal signal sequence, a single membrane-spanning region and a C-terminal cytoplasmic region.