XK is a 444 amino acid protein that spans the membrane 10 times and carries the ubiquitous antigen, Kx, which determines blood type. The XK (X-linked Kx blood group)-related gene family are homologs of XK. XKR6 (XK-related protein 6) is a 641 amino acid multi-pass membrane protein that likely is a component of the XK/Kell complex of the Kell blood group system. The gene encoding XKR6 maps to human chromosome 8, which is made up of nearly 146 million bases and encodes about 800 genes. There are two isoforms of XKR6 that are produced as a result of alternative splicing events.