Fractalkine is the sole member of the CX3C chemokine subfamily with three amino acids between the CI and C2 cysteines of the chemokine domain. It is unique among members of the chemokine family in that it exists in a membrane-bound form with the chemokine domain presented on top of a long mucin stalk. The chemokine domain of fractalkine can mediate the chemotaxis of monocytes. T cells, microglia and NK cells. The chemokine domain of fractalkine is a ligand for the chemokine receptor CX3CR1 and the cytomegalovirus receptor US28. Immobilized forms of fractalkine that include the mucin stalk can mediate the tight adhesion of CX3CRI-expressing cells under flow conditions.