XKR4 (XK-related protein 4) is a 650 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 XKR4 maps to human chromosome 8, which is made up of nearly 146 million bases and encodes about 800 genes. Single nucleotide polymorphisms (SNPs) of the XKR4 gene are associated with iloperidone efficacy, a antipsychotic drug that is used to treat schizophrenic patients.