The human neurotropic polyomavirus JC, JC virus (JCV), infects the majority of human population during early childhood and establishes a latent/persistent infection for the rest of the life. JCV is the etiologic agent of the fatal demyelinating disease of the central nervous system, progressive multifocal leukoencephalopathy (PML) that is seen primarily in immunocompromised individuals. In addition to the PML, JCV has also been shown to transform cells in culture systems and cause a variety of tumors in experimental animals. Moreover, JCV genomic DNA and tumor antigen expression have been shown in a variety of human tumors with CNS origin. Similar to all polyomaviruses, JCV encodes for several tumor antigens from a single transcript of early coding region via alternative splicing. There is little known regarding the characteristics of JCV induced tumors and impact of DNA damage induced by radiation on viral tumor antigen expression and growth of these cells. Here we analyzed the possible impact of ionizing radiation on transformed phenotype and tumor antigen expression by utilizing a mouse medulloblastoma cell line (BSB8) obtained from a mouse transgenic for JCV tumor antigens. Our results suggest that a small subset of BSB8 cells survives and shows radiation resistance. Further analysis of the transformed phenotype of radiation resistant BSB8 cells (BSB8-RR) have revealed that they are capable of forming significantly higher numbers and sizes of colonies under anchorage dependent and independent conditions with reduced viral tumor antigen expression. Moreover, BSB8-RR cells show an increased rate of double-strand DNA break repair by homologous recombination (HR). More interestingly, knockout studies of JCV tumor antigens by utilizing CRISPR/Cas9 gene editing reveal that unlike parental BSB8 cells, BSB8-RR cells are no longer required the expression of viral tumor antigens in order to maintain transformed phenotype.