CX3CL1 is the only member of the CX3C chemokine family that is constitutively expressed in endothelial and epithelial cells, lymphocytes, neurons and microglial osteoblasts. CX3CL1 exists in two forms: as a membrane-anchored form of approximately 100 kD or as a shed 80-95 kD glycoprotein. The chemokine domain is roughly 8 kD, but it migrates at about 13 kD. Its membrane bound form promotes the firm adhesion of rolling leucocytes onto the vessel wall, while its soluble form serves as a potent chemoattractant for CX3CR1-expressing cells such as NK cells, CD14+ monocytes, cytotoxic effector T cells, B cells, neurons, microglia, smooth muscle cells, and tumor cells. The CX3CL1/CX3CR1 axis plays an important role in the pathogenesis of several inflammatory diseases such as allergic pathologies, rheumatoid arthritis, atherosclerosis and tumorigenesis.