Aim 2: Develop risk prediction tools based on viral-host interactions that predispose young SOT and hematopoietic stem cell transplant patients to Epstein-Barr virus disorders/post-transplant lymphoproliferative disorder (EBV disorders/PTLD): We hypothesize that susceptibility to EBV disorders/PTLD is influenced by the interaction of high-risk EBV subtypes with host factors like age, immune maturation and intensity of immunosuppression. In the above context, "EBV disorders" refers to non-malignant EBV disease as well as sustained elevation of viral loads in the absence of PTLD. We will identify high-risk viral subtypes and age-related differences in host immunity that interact to increase susceptibility to EBV disorders/PTLD in young patients. Important clinical applications are: 1) knowledge of host susceptibility factors (age and immune maturation) at the time of transplant may assist choice of less intensive immunosuppression to reduce risk of developing EBV (e.g., avoidance of T cell depletion therapies, primary prevention) and promote use of EBV prophylaxis in at risk patients (secondary prevention); 2) an EBV genotype panel will be developed as a clinical tool for detecting high risk subtypes in patients with EBV. This will help identify patients exposed to EBV who should receive aggressive therapy for EBV and/or PTLD (personalized therapy). This innovative risk stratification will enable personalized immunosuppression strategies and strategies to prevent and treat EBV/PTLD.