RAG1 and colitis: Lastly, to test whether the functional defect in CD4cre:PP4f/f Treg cells was the predominant cause for the spontaneous colitis in the CD4cre:PP4f/f mice, we co-transferred WT or CD4cre:PP4f/f Treg cells with WT CD4+CD45RBhigh T cells into RAG1-/- recipients (Figure 7D).