Th17 preponderance has been inferred by the presence of IL-17 as a relevant proinflammatory mediator, and IL-17 has gained a growing interest, both as a key pathogenic element and also as a potential therapeutic target in rheumatic diseases including rheumatoid arthritis [218–220] and spondyloarthritis [221] including among them psoriatic arthritis [222, 223]. Here, IL17A is linked to psoriatic arthritis.