TH17 cells are a subset of interleukin-17 (IL-17)-secreting T-helper (TH) cells implicated in the pathogenesis of multiple sclerosis (MS), rheumatoid arthritis, juvenile idiopathic arthritis (JIA), and psoriasis1,2, whose differentiation is regulated by the transcription factor RAR-related orphan nuclear receptor gamma (RORγt)3. The gene discussed is IL17A; the disease is myeloid sarcoma.