This is an open label, inpatient-outpatient prospective observational study to determine whether the inter-patient variability in mycophenolic acid (MPA) pharmacokinetics and exposure, adverse events and clinical response in pediatric transplant patients (ages 2-18 years) is associated with identifiable pharmacogenetic factors. Specific aims: 1.) To determine whether common polymorphic variations in the uridine diphosphate-glucuronosyltransferases (UGTs) are associated with inter-individual variability in MPA pharmacokinetics and exposure (by affecting MPA metabolism). 2.) To determine whether common polymorphic variations in the uridine diphosphate-glucuronosyltransferases (UGTs) are associated with inter-patient differences in the incidence of adverse events (by affecting the formation of the acyl-glucuronide metabolite). Enrolled subjects will have been receiving mycophenolate mofetil as part of their clinical standard of care. It is anticipated that the clinical portion of the study will last up to 4 hours at one study visit to include one pharmacogenetic blood sample and 4 pharmacokinetic blood samples collected out to 3 hours post-dose. Safety data to be collected will include standard of care physical exam and safety laboratory tests as well as data on adverse events and clinical outcomes.