XK is the Kell blood group 'precursor substance'. This protein has structural characteristics of prokaryotic and eukaryotic membrane transport proteins. The Kell blood group is constituted by two covalently linked antigens at the surface of red blood cells, Kell and KX. Whereas Kell is a metalloprotease with demonstrated in vitro enzymatic activity, the role of KX thereon, and/or alone, remains unknown, although its absence is linked to the McLeod syndrome, a neuroacanthocytosis. KX appears to be required for proper synthesis or presentation of the Kell antigens on the red blood cell surface.