IL-23 is important in the development of T cell-dependent colitis (178), yet IL-23-dependent colitis does not require IL-17 secretion by T cells, because CD4+ CD45RBhi T cells cannot induce colitis in Il23a−/− Rag1−/− recipients even though intestinal IL-17 is unaffected by the absence of IL-23 (179). This evidence concerns the gene CD4 and colitis.