Given the role of met-enkephalin in modulating adaptive immune cell reactivity (Zagon and McLaughlin, 1991; Zagon et al., 2002; Malendowicz et al., 2005; Donahue et al., 2009), reduced serum enkephalin levels in MS patients may promote immune cell proliferation and drive immune-mediated flares. Here, MET is linked to myeloid sarcoma.