This study will determine the best location to biopsy the gastrointestinal (GI) tract for early and accurate diagnosis of GI graft-versus-host-disease (GVHD). (A biopsy is the surgical removal of a small piece of tissue for examination under the microscope.) GVHD is a life-threatening complication of stem cell transplantation in which the donor s immune cells destroy the patient s healthy tissues. It most commonly affects the skin, liver and GI tract. This study will establish where to best biopsy tissue from the GI tract and study the tissue to try to explore how GI GVD occurs and how it may be possible to better diagnose and treat it.