This study will examine how people differ in the way their bodies process and eliminate mycophenolate mofetil (MMF), a drug that is used to treat problems affecting the eye and immune system and to prevent organ rejection in transplant patients. MMF is metabolized by a group of enzymes called UGTs, each of which is made by a different gene. This study will investigate whether people with different UGT genes differ in how well their bodies use and remove MMF. The results may help scientists learn the best way to give MMF to patients.