Heart transplantation is an advantageous procedure for selected patients with end-stage heart failure. All patients remain on lifetime calcineurin immunosuppressive medications such as cyclosporine or tacrolimus to prevent rejection and extend graft function. Currently, the common practice in managing these patients has been titration of immunosuppressive medications to establish trough levels and clinical response. However, despite the diligent efforts to balance "adequate" immunosuppressive levels with prevention of end-organ toxicity or opportunistic infection; patients still experience acute rejection.