RAG1 and colitis: To achieve this goal, we crossed IRF4 germline deficient mice onto the Rag1-deficient background and elicited colitis in these mice upon naïve CD4+ T cell transfer into Rag1−/−Irf4−/−, Rag1−/−Irf4+/− or Rag1−/−Irf4+/+ mice with the last two groups serving as IRF4-sufficient control animals (termed Rag1−/− mice) (33, 34, 43).