The herpes simplex virus type 1 (HSV-1), which affects almost two-thirds of the world’s population and is generally associated with oral herpes, may cause painful cold sores or fever blisters around the mouth. Yet, when genetically engineered to fight cancer, the virus may also play an important role in treating advanced melanoma, skin cancer that has spread to other parts of the body.

These are the findings of phase 1-2 clinical trial results recently presented by Keck Medicine of USC at the 2025 American Society of Clinical Oncology annual meeting and published in the Journal of Clinical Oncology.

In the study, patients were treated with a genetically modified HSV-1 in combination with an immunotherapy (nivolumab). By the end of the clinical trial, one-third of the participants had their tumors shrink by at least 30%, and nearly one out of six patients had tumors completely disappear.

Read more about this study.