Given the proinflammatory functions of Th9 cells in allergic inflammation, autoimmunity and tumor immunity, the identification of Foxo1 as a key transcription factor that dictates the development and effector functions of Th9 and IL-9-producing T cells could prove beneficial in designing targeted therapies aimed at alleviating the course of autoimmune diseases and anti-cancer therapy. Here, IL9 is linked to Autoimmunity.