There have been some early studies of the use of Cellcept, but none have proven a relationship between the blood level of the drug and how well it works. More also needs to be known about how this drug works with other anti-rejection drugs and how it works in boys and girls. This study will look more closely at proper dosing, how Cellcept works with other anti-rejection medications, side effects, and any differences in how this medicine works in boys and girls.