• The following outcome measures will be performed at study visit 2 (2-3 days post-transplant), study visit 3 (6-9 days post-transplant), and study visit 4 (3-6 months post-transplant):
  
• Drug Measurement of MPA and MPA-G
  
• IMPDH Assay
  
• Mitogenesis Assays and
  
• Determination of CD25 Expression.
  

• Concomitant medications and MPA (CellCept) drug diary (study visits 1-4)
  
• Physical Exam (study visits 1-4)
  
• Safety Laboratory tests (study visits 1-4)
  
• Dietary Monitoring (study visits 2,3 & 4)
  
• Adverse Event Reporting (study visits 1-4).
  

This is an open-label, inpatient-outpatient population pharmacokinetic-pharmacodynamic (PK-PD) study of mycophenolic acid (MPA) in pediatric subjects (age 2-17 years) who have had a recent kidney transplant, as well as during the stable renal transplant period when on a stable oral regimen of MPA. The primary objective of this study is to develop population pharmacokinetic-pharmacodynamic (PK-PD) models for mycophenolic acid (MPA) and mycophenolic acid glucuronide (MPAG) in order to improve individualized pediatric dosing. Subjects will have been receiving CellCept as part of their clinical standard of care. It is anticipated that the clinical portion of the study for each patient will be approximately six months post renal transplant with four study days: a screening visit pre-transplant, two 10-hour inpatient days at 2-3 and 6-9 days post-transplant, and one (up to 10 hour) outpatient visit at 3-6 months post-transplant. Pharmacokinetic and pharmacodynamic measurements will be conducted at various time points (up to 9 hours post dose) on study days 2, 3, and 4. Safety data to be collected will include physical examinations, measurement of vital signs, and laboratory assessments, as well as data on adverse events and clinical outcomes.