The cytokine IL-23 contributes to the polarization of naïve CD4+ T cells into IL-17-producing Th17 cells, while IL-12 in turn induces the transformation of the highly plastic Th17 cells into IFN-γ-producing Th1/17 cells.38 The distinctive presence of activated Th1/17 cells in CD-associated fistulas suggests that the monoclonal antibody ustekinumab, directly targeting the common p40 subunit of IL-12 and IL-23, could be effective in patients with perianal CD (pCD). Here, IL17A is linked to Cowden disease.