COLLECT is a monocentric, prospective, observational study, which aims to assess the association between changes in the intestinal microbiota and the incidence of gastrointestinal graft-versus-host diseases (GvHD). Patients admitted for performance of an allogeneic hematopoietic stem cell transplantation (HSCT) or patients with a first diagnosis of an acute myeloid leukemia (AML) will be enrolled and stool samples will be analyzed using next-generation sequencing. In addition to stool, blood and urine samples will be collected for cytokine and 3-indoxylsulfate analysis.