It is difficult to directly assess the relative importance of gastrointestinal radiation sensitivity versus increased GVHD in Ceacam1 2/2 allo-BMT recipients, as radiation-induced gut damage may both be directly manifested in intestinal pathology, yet transmural migration of bacterial superantigens is an important first step for the initiation of GVHD [1, 39, 40] , and increased damage to the intestines of Ceacam1 2/2 mice may thus amplify the development of GVHD in these mice, and also explain in part the specifically increased large intestinal GVHD we observed.