A mutant herpesvirus that has a mutated gene that encodes a mutant infected cell protein 0 (ICP0) can be used in therapeutic methods as well as in diagnostics and research experiments. The encoded mutant ICP0 protein can be altered in one or more regions of ICP0 that are substantially conserved between two or more herpesviruses and/or within a phosphorylation region. The mutant herpesvirus can be substantially avirulent and immunogenic.