RAG1 and colitis: In the absence of Tregs, adoptive transfer of naive CD4+ T cells (CD45RBhiCD25−CD4+) into Rag1−/− mice induced severe colitis, as indicated by weight loss (Fig. 3B), shortened colon (Fig. 3C), damaged tissues (Fig. 3D), and greatly increased pro-inflammatory IFN-γ+CD4+ T, but not IL-17A+CD4+, cells in the colon and mLN (Fig. 3E), whereas cotransfer of Foxp3YFP-Cre Tregs, but not Sf3b1K700Efl/+/Foxp3YFP-Cre Tregs, prevented these severe colitis phenotypes in Rag1−/− mice (Fig. 3, B to E).