About 13% of adult heart transplant recipients do not survive to one year and a major cause of death is acute cellular allograft rejection.1,2 According to the 2009 annual United States data published from the International Society for Heart Lung Transplantation Registry, acute rejection occurs in 25 – 35% of transplant recipients within the first year following transplant surgery.3 In order to detect the early stages of rejection so that more aggressive and early immunosuppressant therapy can be initiated, frequent biopsies of heart tissue are performed (typically, weekly or every other week in the first 3 months and then monthly or every other month during the first year). Although endomyocardial biopsy (EMB) is not a perfect “gold standard” for a correct diagnosis of acute allograft rejection, it is considered the best available test and thus, it is the current standard practice. Unfortunately, EMB is an invasive and costly procedure that is not without risk.4,5 If a simple noninvasive biomarker could be identified to detect the early stages of acute rejection, it might be possible to reduce the number of invasive biopsy procedures and to initiate earlier therapy that might prevent death from severe rejection.