These genes are each composed of 5 exons and code for the beta-subunits of voltage-gated sodium channels. Voltage-gated sodium channels are responsible for the propagation of action potentials in striated muscle tissue and neuronal cells. Mutations in these genes have been associated with Brugada syndrome, defects in cardiac conduction, atrial fibrillation, Sudden Infant Death syndrome (SIDS) and LongQT syndrome (LQTS).