To address the issues of whether leukocytes bearing TJ proteins enter the CNS, and the possible origin of these proteins, initial studies were performed to identify leukocytes harboring the TJ protein CLN-5—a determinant of the BBB [53]—in the blood and spinal cord of wild-type mice with experimental autoimmune encephalomyelitis (EAE), an animal model of MS [54–56]. The gene discussed is CLN5; the disease is myeloid sarcoma.