May be involved in forming the receptor site for cardiac glycoside binding or may modulate the transport function of the sodium ATPase. Regulatory subunit of the sodium/potassium-transporting ATPase which is composed of a catalytic alpha subunit, an auxiliary non-catalytic beta subunit and an additional regulatory subunit. Highest levels expressed in the kidney and spleen. Restricted to the basolateral membrane in renal epithelial cells and varies in its level of expression along the nephron. Belongs to the FXYD family.