Strikingly, active lesions contain a greater abundance of IL-17-producing CD4+ and CD8+ T cells than inactive MS lesions do.249 IL-17 was also shown to promote BBB disruption and facilitate the migration of Th17 cells into the CNS, where they directly contribute to tissue damage247; reviewed in ref.250 These emerging data highlight the relevance of IL-17 and Th17 cells in MS, completely changing the established paradigm of Th1-driven MS immunopathogenesis (Fig. 2). This evidence concerns the gene IL17A and myeloid sarcoma.