An oncolytic herpes simplex virus type 1 mutant, named HF10, has been isolated and evaluated for anti-tumor efficacy in syngeneic immuno-competent mouse models. We have found that the mutant virus can very effectively treat cancer, and that all of survived mice acquire resistance to rechallenge of the tumor cells. Since a number of studies have shown that HF10 is effective and safe for use in localized or peritoneally disseminated malignant tumors of non-neuronal origin, phase I/II clinical trials using HF10 have been initiated for patients with metastatic breast cancer. Preliminary data from the clinical trials are encouraging.