Top panels: MRI brain images of a patient with post-transplant primary central nervous system lymphoproliferative disorder associated with the Epstein-Barr Virus (EBV). Middle panels: Non-reactive control tissue (left), and stereotactic core brain biopsy of this patient's tumor demonstrating evidence of constitutive EBV thymidine kinase gene expression by in situ RT-PCR (right). Bottom panels: MRI brain images of the patient one year after initiating treatment with high dose anti-viral therapy (ganciclovir and zidobudine) show only residual scar tissue. The patient has now been disease-free for three years. For details, see the article by Roychowdhury et al. on page 965 of this issue.