RAG1 and colitis: Furthermore, in a model of T cell transfer-induced colitis, using a mast cell-deficient mouse strain c-kitw-sh/w-sh backcrossed onto a RAG1−/− background, it was demonstrated that mice reconstituted with wt bone marrow-derived mast cells (BM-MC) prior to co-transfer of naïve CD4+ T cells and Tregs had a much lower colitis score than mast cell-deficient mice that had either not been reconstituted or reconstituted with Areg−/− BM-MC.