<p>This group of proteins is found in sodium, potassium, and calcium ion channels proteins. The proteins have 6 transmembrane helices in which the last two helices flank a loop which determines ion selectivity. In some Na channels proteins the domain is repeated four times, whereas in others (e.g. K channels) the protein forms a tetramer in the membrane. A bacterial structure of the protein is known for the last two helices but is not included in the Pfam family due to it lacking the first four helices. </p>