CX3CL1 and neurodegenerative disease: Here we will discuss age-dependent changes in fractalkine (FKN, CX3CL1) signaling, on the balance among soluble (sCX3CL1) vs. membrane-bound CX3CL1 (mCX3CL1) forms, and how changes in this balance contributes to switching microglia from an anti-inflammatory to a pro-inflammatory status, a key mechanism likely involved in chronic CNS inflammation and neurodegenerative diseases.