CX3CR1 is a 40 kD, G-protein coupled receptor, with seven transmembrane regions. CX3CR1 is expressed by resident and alternatively activated macrophages (M2), a subset of monocytes, dendritic cells (DCs), NK cells, a subset of memory T cells, and mast cells. CX3CR1 is involved in cell recruitment during inflammation and participates in cell adhesion and extravasation from blood vessels. Its ligand is CX3CL1, also known as fractalkine or neurotactin. CX3CR1 is also a coreceptor for HIV1 and variations in this gene leads to increased susceptibility to HIV. In the brain, it is expressed by glial cells, which interact with CX3CL1 expressed by neurons.