The objective of this prospective observational single center study is to investigate donor-derived cell-free DNA (ddcfDNA), peripheral blood platelet mRNA, peripheral blood extracellular vesicle mRNA, and peripheral blood leukocyte mRNA expression in recognition of clinically significant endomyocardial biopsy (EMB) proven acute rejection in human heart transplant recipients. In detail, the objective is to develop novel biomarkers and liquid biopsies for diagnosis, prognosis, and targeted molecular therapy for primary graft failure, ischemia-reperfusion injury, acute rejection, and development of late graft failure and cardiac allograft vasculopathy, and for monitoring immunosuppression after heart transplantation.