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ABSTRACTThe seven-transmembrane receptor CX3CR1 is a specific receptor for the novel CX3C chemokine fractalkine (FKN) (neurotactin). In vitro data suggest that membrane anchoring of FKN, and the existence of a shed, soluble FKN isoform allow for both adhesive and chemoattractive properties. Expression on activated endothelium and neurons defines FKN as a potential target for therapeutic intervention in inflammatory conditions, particularly central nervous system diseases. To investigate the physiological function of CX3CR1-FKN interactions, we generated a mouse strain in which the CX3CR1gene was replaced by a green fluorescent protein (GFP) reporter gene. In addition to the creation of a mutant CX3CR1locus, this approach enabled us to assign murine CX3CR1 expression to monocytes, subsets of NK and dendritic cells, and the brain microglia. Analysis of CX3CR1-deficient mice indicates that CX3CR1 is the only murine FKN receptor. Yet, defying anticipated FKN functions, absence of CX3CR1 interferes neither with monocyte extravasation in a peritonitis model nor with DC migration and differentiation in response to microbial antigens or contact sensitizers. Furthermore, a prominent response of CX3CR1-deficient microglia to peripheral nerve injury indicates unimpaired neuronal-glial cross talk in the absence of CX3CR1.