Mycophenolate mofetil (MMF) is commonly used for GVHD prophylaxis, particularly in the setting of nonmyeloablative allogeneic SCT. It is also being used with increasing frequency to treat GVHD and may be more effective in combination with steroids than other therapies [26] . However, increasing evidence suggests that variable pharmacokinetics may be critical to efficacy of MMF.