Galectin-9, widely expressed in many tissues, regulates Th1 cells and induces their apoptosis through its receptor, T-cell immunoglobulin mucin 3 (TIM-3), which is mainly expressed on terminally differentiated Th1 cells. Type 1 diabetes is a Th1-dominant autoimmune disease that specifically destroys insulin-producing cells. To suppress the Th1 immune response in the development of autoimmune diabetes, we overexpressed galectin-9 in NOD mice by injection of a plasmid encoding galectin-9.