In addition, an increase in Th17 cells, the main IL17-secreting cells, is observed in atopic dermatitis, psoriatic arthritis, and the six extraintestinal manifestations mentioned above and is thought to be involved in the pathogenesis of these diseases, which likewise play an important role in the pathogenesis of IBD (33, 34). The gene discussed is IL17A; the disease is psoriatic arthritis.