EBV-associated post-transplant lymphoproliferative disease (PTLD) is the most common malignancy in children after transplant. Diagnosis and effective treatment of the EBV-associated cancer is hampered by our inability to determine which children are at risk of developing these cancers and to detect the cancer at an early stage. In this study, we plan to test new "biomarkers" in the blood of children that will tell us very early on if the child is at risk of developing the EBV-associated cancer or if the cancer is present. These studies provide new opportunities for detection, diagnosis, and treatment of children with EBV-associated, post-transplant cancer.