In a T cell-mediated colitis model in which adoptive transfer of CD45RBhi T cells (naïve CD4 T cells depleted of Tregs) to immunodeficient recipient mice resulted in the generation and intestinal trafficking of Teff cells to cause colitis (31–33), Rag2−/− mice that received naïve Gpr15-KO T cells were protected from developing colitis. The gene discussed is CD4; the disease is colitis.