Anti-X123 antibodies are protein reagents that detect specific antigens. The X123 antigen is another term for the human protein endosomal transmembrane epsin interactor 1, encoded by the ENTREP1 gene. The protein is known to function as a activator of the E3 ubiquitin protein ligase ITCH in the ubiquitination of the CXCL12-activated CXCR4 receptor. Canonically, it has an amino acid length of 450 residues and a mass of 49.7 kilodaltons. Its subcellular localization is in the cell membrane and it is notably expressed in muscle. The X123 protein is a member of the ENTREP protein family.