CX3CR1 is a specific receptor for fractalkine/neurotactin (CX3CL1), a transmembrane molecule having an N-terminal chemokine domain with the CX3C motif. Membrane-bound fractalkine induces a rapid integrin-independent firm adhesion of leukocytes expressing CX3CR1 without involving any other adhesion molecules or G protein signaling; while soluble fractalkine induces vigorous chemotaxis of leukocytes expressing CX3CR1 though a pertussis toxin-sensitive signaling pathway. Thus, CX3CR1 has a dual function as an adhesion molecule and a signal transducer for leukocyte migration. Abundant expression of CX3CR1 in microglia and that of fractalkine in neurons also suggests important roles in the brain.