It regulates the expression of several key Th17 genes, including IL17A (encoding interleukin 17A), IL17F (encoding interleukin 17F), CCR6 (encoding C–C motif chemokine receptor 6), and CXCL13 (encoding C-X-C motif chemokine ligand 13), through SMAD regulation of TGF signal transduction [35], ultimately regulating human Th17 differentiation and mediating the occurrence of autoimmune diseases such as rheumatoid arthritis (RA) and psoriasis [21]. The gene discussed is IL17A; the disease is psoriasis.