XPLAC Background Information Kell and XK are two covalently linked plasma membrane proteins that constitute the Kell blood group system, a group of antigens on the surface of red blood cells that are important determinants of blood type and targets for autoimmune or alloimmune diseases. XK is a 444 amino acid protein that spans the membrane 10 times and carries the ubiquitous antigen, Kx, which determines blood type. XK also plays a role in the sodium-dependent membrane transport of oligopeptides and neutral amino acids. XPLAC is a 462 amino acid member of the XK family that is expressed mostly in placenta and adrenal gland. The XPLAC gene is located on the X chromosome at position q22.1.
