Following the discovery that the CD45RBlow population comprised a population of CD4+CD25+ regulatory T cells that could cure colitis in SCID or RAG1-deficient recipients of CD4+CD45RBhigh T cells [138], the T-cell transfer model of colitis was further refined by reconstitution of SCID mice with CD25-depleted CD4+ cells [139]. Here, RAG1 is linked to colitis.