CXCL12 and CXCR4 are constitutively expressed and widely detected in the CNS, especially in microglia, and have been shown to play important roles in the physiology of the brain and in pathological conditions, such as stroke, multiple sclerosis (MS), and some neurodegenerative diseases [49–51]. The gene discussed is CXCR4; the disease is myeloid sarcoma.