T helper 17 cells have been shown to play an important role in multiple immune-mediated inflammatory conditions including rheumatoid arthritis, experimental autoimmune encephalomyelitis, inflammatory bowel disease, and dry eye disease.8,19 T helper 17 cells are characterized by their ability to produce several cytokines including GM-CSF, which has drawn attention as a key component of disease pathogenesis in a number of Th17 cell–associated conditions. The gene discussed is CSF2; the disease is rheumatoid arthritis.