Immunohistochemical detection of polyomavirus oncoprotein, T-antigen in human brain tumors. JC Virus is a human neurotropic polyomavirus that infects more than 80% of the adult population. Under immunosuppressive conditions, the virus is responsible for causing Progressive Multifocal Leukoencephalopathy (PML), a subacute fatal demyelinating disease of the brain. The early genome of this virus encodes the regulatory protein, T-antigen, which plays an important role in control of the viral lytic cycle in brain cells of PML patients and possesses oncogenic potential in several experimental animals. Using an antibody that recognizes T-antigen, we examined sections of different types of human glial tumors, including oligodendroglioma, anaplastic, oligodendroglioma, pilocytic astrocytoma, diffuse fibrillary astrocytoma, Glioblastoma Multiforme and ependymoma for the production of the viral protein. The cover image shows immunopositive nuclei in neoplastic cells from Glioblastoma Multiforme. For more information, please see the article by Del Valle et al. on p. 4287 of this issue.