Gastro-intestinal graft versus host disease (GVHD) is a major source of morbidity and mortality amongst allogenic hematopoietic stem cell transplantation (Allo-HSCT). The diagnosis is based on histological findings that involve colonic biopsies with a risk of bleeding, especially in case of thrombocytopenia. Moreover the diagnosis is frequently made at a clinical stage of the disease, after the appearance of gastro-intestinal symptoms. Endo-microscopy is a novel endoscopic technique that allow "optical biopsies" during a conventional endoscopy and has proved its efficiency in several indications. In a pilot study the investigators showed that it had good sensibility and sensitivity compared to histology as gold standard. Therefore this study aim to identify endo-microscopic criteria allowing the early diagnosis of GHVD before its clinical manifestations.