We will conduct an open-label, PK study in stable CF lung transplant patients. Our study will also evaluate non-CF patients as the control group. Each patient will have three biweekly PK study visits. On the PK study days, patients will be admitted to the Michigan Clinical Research Unit (MCRU) at the University of Michigan Health Center. After administration of morning dose of MMF, serial blood samples will be collected at the following time intervals: immediately before and 0.5, 1, 1.5, 2, 4, 6, 9, and 12 hours after dosing. Serum creatinine and serum albumin will also be drawn. Vial of blood will be also collected to measure the function of IMPDH. Standardized meals will be provided on PK study days. Serum concentrations of total MPA and MPAG will be determined by a validated liquid chromatography with tandem mass spectrometry method.10