Because of this concern, we turned our attention to mutations in the γ134. 5 gene (5), whose product, ICP34. 5, promotes infection of nondividing cells and inhibits apoptosis by infected cells (1, 3). Null mutants of γ134. 5 do not replicate within healthy adult neurons or cause encephalitis, but grow in permissive, actively cycling cells. Crucially, HSV-1 mutants deficient for ICP34. 5 retain their sensitivity to acyclovir and ganciclovir and can destroy malignant glioma cells in culture and in vivo. This mutation would therefore appear to preserve the oncolytic activity of the virus while abolishing its toxicity, but the possibility remained that secondary viral or cellular mutations could suppress its effects. Seeking an additional mutation that could be engineered into ICP34. 5-deficient strains to provide still greater assurance of safety, we chose to study hrR3 (2). This strain of HSV-1 carries a mutation in the UL39 gene, which encodes the large subunit of HSV ribonucleotide reductase (RR). As with TK, mammalian RR is elevated in tumor cells, so hrR3 might be predicted to grow preferentially within tumor cells. Further, RR-negative mutants had been shown to have decreased neurovirulence and increased sensitivity to acyclovir and ganciclovir, and 2 studies independently confirmed the efficacy of RR-negative mutants in treating malignant glioma (6, 7).