Other biopsies obtained at baseline, following the conditioning regimen as well as before and after therapy for GI GVHD will be used to achieve the secondary objective of understanding the immune response underlying GI GVHD. A comprehensive evaluation of the inflammatory milieu of gut tissues will be achieved using immunohistochemistry and flow cytometric immunophenotyping of mucosal mononuclear cells and microarray, qRT-PCR and ELISA of mucosal cytokines. Other secondary objectives include the evaluation of serum proteomic pattern analysis, serum citrulline, fecal calprotectin and 18F-FDG PET/CT for the noninvasive diagnosis of GI GVHD.