Heart transplant is the only effective treatment for people with advanced heart failure. Post-transplant pharmacological therapies are of fundamental importance for the survival of individuals after surgery: although considerable progress has been made for combined immunosuppressive therapies, acute cellular and especially non-cellular rejection still represents a great challenge for doctors. To verify the absence of the first signs of acute rejection, the analysis of numerous cardiac biopsies (EMB endomyocardial biopsies) is necessary during the first 12 months following the transplant. Thanks to these scheduled checks, doctors are able to identify the first symptoms of possible chronic rejection and reduce its episodes. Since the analysis of biopsies is also based on subjective interpretations, cases of erroneous conclusions are frequent. The researchers of this study aim not only to analyze the biopsies according to the current best clinical practice, but also to evaluate how much anti-rejection drug is actually contained within them. This is an analysis that is still little used for this type of transplant, which could provide very useful information to doctors. The researchers will focus their attention on one drug in particular, tacrolimus, abbreviated to TAC. The amount of drug measured in biopsies will be compared with that measured in whole blood samples and in particular blood cells (peripheral blood mononuclear cells: PBMC). The genetic characteristics of each person play an important role in the success of treatment with the drug. To best interpret the results, all participants will be asked to take a blood sample to identify some characteristics of their DNA that could influence the outcome of tacrolimus therapy.