CXCR4 and neurodegenerative disease: The chemokine receptors CCR2, CCR5, CXCR2, CXCR3, CXCR4 and CX3CR1 are constitutively expressed in the human brain, and their expression is enhanced under pathological conditions, including stroke[13,24] and neurodegenerative diseases like HIV-associated dementia (HAD)[4,37] and Alzheimer’s disease (AD)[38].