1. Department of Neurosurgery, Johns Hopkins University School of Medicine, Baltimore, MD 21287, USA.
2. Department of Oncology, Johns Hopkins University School of Medicine, Baltimore, MD 21287, USA.
3. Department of Neurosurgery, Seoul National University College of Medicine, Seoul 110-744, South Korea.
4. Department of Cancer Immunology, Johns Hopkins University School of Medicine, Baltimore, MD 21287, USA.