CX3CL1, also named neurotactin, is a member of the delta chemokine subfamily that contains a novel C-X3-C motif. Unlike other known chemokines, CX3CL1 is a type 1 membrane protein containing a chemokine domain tethered on a long mucin-like stalk. The extracellular domain of CX3CL1 can potentially be released as a soluble protein by proteolysis at the conserved dibasic motif proximal to the transmembrane region. With the exception of the stalk region, rat CX3CL1 shares a high degree of amino sequence homology (83% sequence identity) with human and mouse CX3CL1. CX3CL1 is expressed in various tissues including heart, brain, lung, kidney, skeletal muscle, and testis. In rat brain, CX3CL1 expression was found to be localized principally to neurons. The expression of CX3CL1 was also reported to be up‑regulated on activated endothelial cells. Membrane‑bound CX3CL1 has been shown to promote adhesion of leukocytes. The soluble chemokine domain of human CX3CL1 was reported to be chemotactic for T cells and monocytes while the soluble chemokine domain of mouse CX3CL1 was reported to chemoattract neutrophils and T‑lymphocytes but not monocytes. CX3CR1, previously named V28 or chemokine beta receptor‑like 1, has been found to be a specific receptor for CX3CL1. In addition, US28, a 7TM receptor encoded by human cytomegalovirus that binds multiple CC chemokines, has also been shown to bind fractalkine with high-affinity.