Accumulating evidence suggests that TH17 cells become pathogenic through the IL-23–IL-23R axis and play crucial roles in development of various autoimmune diseases, including psoriasis.8, 56, 57 However, how these TH17 cells acquired the pathogenicity in vivo and to what extent the microenvironment of diseases contributes to this process remain to be defined. Here, IL23A is linked to psoriasis.