The accelerated early mortality of Ceacam1 2/2 allo-BMT recipients, together with increased accumulation of donor T cells in GI tract and mesenteric lymph nodes, but decreased numbers peripheral lymph nodes ( Figure 3C ), led us to ask whether Ceacam1 had differential effects in regulating GVHD target organ damage for various target organs and tissues. In the context of Ceacam1 2/2 recipients, we therefore tested the radiation sensitivity of Ceacam1 2/2 mice used as hosts, by irradiating WT and Ceacam1 2/2 BALB/c mice and assessing survival. Ceacam1 2/2 animals showed increased kinetics of mortality, and in some cases, overall mortality after radiation injury ( Figure 4A ). Similar results were obtained on the B6 background ( Figure 4C ). We then enumerated regenerating and surviving crypts in the small intestine (terminal ileum) at 84 hours after irradiation to assess intestinal radiation damage, and observed that Ceacam1 2/2 mice had fewer regenerating and surviving crypts as compared with WT counterparts ( Figure 4B and D), indicating greater damage to the small intestine across a wide range of radiation doses. It is thus quite likely that our radiation-containing conditioning regimen for transplant recipients also contributes in part to their survival kinetics, selective GVHD target organ damage ( Figure 2G -H), and the selective accumulation of donor T cells in lymphoid tissues and target organs ( Figure 3C ) in these recipients.