This is a retrospective and prospective non-interventional multicenter observational study. Neither diagnostic approaches nor experimental drugs/procedure will be applied and the samples will take place at the same time as the samples will be taken during routinary clinical practice.

The aim of this study is to analyze the immunobiology of Acute Myeloid Leukemia (AML) relapses after allogeneic HSCT for the generation of guidelines and personalized therapeutic pathways.

Background: The identification of leukemic relapses characterized by the "Human Leukocyte Antigens loss" event or by changes in the expression levels of genes involved in antigen presentation and lymphocyte co-stimulation demonstrate how a complete characterization of disease recurrence is necessary to ensure the patient a treatment that is as personalized and targeted as possible, avoiding the choice of sub-optimal or, sometimes, harmful therapies. Unfortunately, the identification of the post-transplant relapse mechanism is now far from being part of an established clinical routine, mainly due to the lack of quick and easy-to-access diagnostic tools and multicentre studies demonstrating the efficacy of a type of personalized therapeutic approach.

The main objective of the study is to favor, exploiting the consolidated collaboration between the Italian transplant centers belonging to the Italian Group of Bone Marrow Transplantation, Hematopoietic Stem Cells and Cell Therapies (GITMO), the implementation in clinical practice of an in-depth biological study of relapses of leukemia in order to allow an increasingly targeted and effective planning of the therapeutic strategies to be implemented, ensuring better patient survival.

The study is divided into two parts:

• data collection, from clinical standard practice, of patients with relapse of the disease which will allow to standardize the therapeutic management process and will allow us to promote innovative clinical studies on acute myeloid leukemia post allogeneic HSCT, with the ultimate aim of improving the survival of these patients;
• biological substudy that will allow to define the role of the tumor microenvironment in promoting or counteracting the phenomenon of relapse by identifying its specific alterations in the various forms of disease relapse.

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.

The cohort of this study will be composed of adult patients with Acute Myeloid Leukemia, who will relapse after allogeneic transplantation from either familial or unrelated donor, regardless of the cellular source of the transplant (bone marrow, mobilized peripheral stem cells or cord blood).

The samples of interest are those taken at the time of post-transplant recurrence at the same time as routine investigations and may be of both medullary blood and peripheral blood, as long as the samples have a percentage of pathological cells equal to at least 5% of cellularity.

Yet to be determined