Some patients with NPC show evidence of infection with the virus that causes infectious mononucleosis, known as the Epstein Barr virus (EBV), before or at the time of their cancer diagnosis. EBV is found in the cancer cells of almost all patients with advanced stage disease, suggesting that it may play a role in causing NPC. Previously, patients have been treated with high-risk NPC using EBV-specific cytotoxic T cells. These cells are grown in the laboratory and taught to recognize and attack EBV infected cells. In the past, patients were either given the cells alone or just after they had received a medication to briefly lower their white blood cell count. In both cases, many patients had their tumors shrink and in some cases completely disappear after being treated with these EBV-specific cytotoxic T cells.