Inclusion Criteria:  Adult patients with Acute Myeloid Leukemia, who relapse after allogeneic transplantation from either family or unrelated donors, regardless of the cellular source of the transplant (bone marrow, mobilized peripheral stem cells or cord blood), who have signed the informed consent of the study; For the coordinating center, all patients who have previously signed informed consent to the "Hematological Neoplasms Biobank" and for which samples, stored in the Biobank are available. For the other centers, all patients who have previously signed an informed consent, aimed at the prior authorization for the storage of samples in the biobank of the aforementioned center, according to centers own practice, and for which samples, stored in a Biobank, are available.  Exclusion Criteria:  Participation in clinical protocols that expressly exclude the possibility of participating in other studies.