There is increasing evidence that Th17 cells and the cytokines they release such as interleukin-17 (IL-17) are important regulators of innate and adaptive immune responses in many Th1- and/or Th2-mediated autoimmune diseases such as rheumatoid arthritis, systemic lupus erythematosus, and allergic asthma [18]. This evidence concerns the gene IL17A and autoimmune disease.