Mast cells are increased in the synovial compartments in RA patients and synovial fibroblasts in AIA produce IL-33, which activates mast cells to release inflammatory mediators including IL-17, IL-13, GM-CSF, MCP-1, and MIP-1α (Robbie-Ryan and Brown, 2002; Table 1). The gene discussed is IL17A; the disease is rheumatoid arthritis.