XKRX (MIM 300684) and XKR3 are homologs of the Kell blood group precursor XK (MIM 314850), which is a putative membrane transporter and a component of the XK/Kell complex of the Kell blood group system (Calenda et al., 2006 [PubMed 16431037]).[supplied by OMIM, Mar 2008].