Recent data examining the dynamic expression of the chemokine CXCL12 at the blood-brain barrier (BBB) indicate that activity of CXCR7, a CXCL12 receptor that sequesters the chemokine intracellularly [9-11], is critical for the entry of infiltrating leukocytes in mice with experimental autoimmune encephalomyelitis (EAE), a murine model for MS [12]. This evidence concerns the gene ACKR3 and myeloid sarcoma.