Subset-specific localization and trafficking kinetics have been recently demonstrated in T cells, with the finding that a subset of highly cytotoxic human effector memory CD8+ T cells were selectively localized within the vasculature and remained there for months in multiple sclerosis (MS) patients who received a drug which inhibits sphingosine-1-phosphate (S1P) dependent tissue egress (45). This evidence concerns the gene CD8A and myeloid sarcoma.