San Francisco ? Researchers said Friday they have found a virus in the prostates of some cancer patients, a remarkable discovery that may suggest disease could play a role alongside genetics and the environment in causing this cancer.

The virus, closely related to one previously found only in mice, was found in cancerous prostates removed from men with a certain genetic defect. The findings open new avenues for studying the most common major cancer among men in the United States.

The researchers, with the University of California, San Francisco, and the Cleveland Clinic, presented the findings at an American Society of Clinical Oncology meeting in San Francisco. They warn that they have not found any links between the virus and prostate cancer, but they are nonetheless excited about prospects for future research.

“We have made a very fascinating discovery never before seen in humans that is very similar to one found in a mammal that causes cancer,” Dr. Eric Klein, of the Cleveland Clinic, said at a news conference. “But we have not proven this virus causes prostate cancer.”

Researchers are not sure how the mouse virus infected people, but suspect it has been passed on genetically for many generations.

“This is a class of virus no one would have looked for in prostate cancer,” said UCSF researcher Joe DeRisi, who developed the so-called “gene chip” that made the discovery. DeRisi’s chip contains 20,000 snippets of vital genetic material from every known virus. It is the same chip that confirmed a previously undiscovered virus in the cold family that caused the SARS outbreak three years ago.

Klein sent samples of 86 cancerous prostates he removed from patients to DeRisi. DeRisi then placed DNA from the cancerous tissues on the chip, and DNA from eight of 20 patients with two copies of a mutated gene matched with DNA from the mouse virus.