XK-related protein 3 (XKR3)

**XK-related protein 3 (XKR3)** is a protein encoded by the XKR3 gene, located on chromosome 22[7]. It is a homolog of the XK protein, the precursor of the Kell blood group system, which is a key antigenic determinant on human red blood cells[1][6][9][10][12]. XK and its homologs are multipass membrane proteins, presumed to function as membrane transporters, but the exact substrate(s) and physiologic function of XKR3 remain unknown[1][6][9][10][12]. The Kell protein and XK protein form a complex on the surface of erythrocytes via a disulfide bond, and disruption of this complex is associated with certain blood group phenotypes and rare disorders such as McLeod syndrome—a neuroacanthocytosis syndrome characterized by abnormal red blood cells and neurological manifestations[11]. While mutations in the XK gene are directly implicated in McLeod syndrome, XKR3 is considered a paralogous protein with unknown clinical significance[1]. XKR3 expression has been observed in several tissues, but unlike the canonical XK, there is no evidence that XKR3 by itself serves as a direct therapeutic target, nor are there known drugs or clinical biomarkers associated with it[13]. The protein is generally classified under "Other" in molecular classification systems due to its putative transporter structure and lack of established function or drug interactions[1][6][9][10][12]. There is no evidence of XKR3 being a target for approved therapeutics or playing a definable mechanistic role in human disease beyond homologous relationships and involvement in similar complexes to XK[1][6][9][10][12].

Other names

XK related 3XKR3XK Kell blood group complex subunit-related family member 3XTGESXRG3MGC57211XTESXK-related protein 3X-linked Kx blood group related 3XK, Kell blood group complex subunit-related family member 3 expressed in testisX Kell blood group precursor-related family member 3