Using immunohistochemical analysis, studies have shown that CXCL12 is constitutively expressed by endothelial cells on the abluminal surface of the BBB, but that CXCL12 production during MS shifts to the luminal site of the endothelium and this change is linked to increased MS severity (101, 217, 218). This evidence concerns the gene CXCL12 and myeloid sarcoma.