Aim 3: Comparison of the relative bioavailability and steady-state pharmacokinetics of 6 tacrolimus formulations in a prospective, 6-way cross-over study including CYP3A5 expressors (n=30) and non-expressor (n=30) transplant patients. Six tacrolimus formulations will be tested and each patient will receive each formulation once. As we proposed to test bioequivalence in the steady-state, patients will receive the test formulations for one week prior to pharmacokinetic evaluation. The pharmacokinetic evaluation will incorporate limited sampling strategies with a focus on fully characterizing the Cmax out to hour 4 post dose. Subsequent PK sampling and trough blood concentrations will be monitored on a daily basis using dried blood spots that the study subjects will collect by themselves at home. The daily monitoring of levels will allow us to specifically address whether the need of additional monitoring upon formulation conversion is necessary. It will be critical that the patients are adherent to their test medication to ensure that they have reached steady state. This will be monitored using test diaries, pill counts and MEMS caps (Medication Event Monitoring System (MEMS), AARDEX Corp, Palo Alto, CABioequivalence will be tested using scaled average bioequivalence metrics and analysis of variance as appropriate and intra-individual variability of the formulations will be compared. This will also include the analysis of potential period and sequence effects. A combination of limited sampling strategy and dry spot analysis in combination with population pharmacokinetic modeling will be utilized to fully characterize the PK profile of these formulations. In addition, the same study will be conducted in 36 pediatric patients.