A crucial role of IL23/Th17 axis in the pathogenesis of psoriasis was proposed based on several recent studies: (i) dermal IL-17-producing CD4+ T cell and γδ T cell infiltrate as well as (ii) IL-17-producing CD8+ T cells within psoriatic epidermis; (iii) high expression levels of IL-23, IL-17, and IL-22 in psoriatic lesional skin; (iv) high serum levels of IL-22 and IL-17 that correlated with disease severity score [4], [5], [17]–[21]. This evidence concerns the gene IL23A and psoriasis.