The marker, called B7-H3, is a protein that's involved in the body's immune system.

Mayo Clinic doctors including Timothy Roth, MD, report that B7-H3 is more abundant in prostate cancers than in normal prostate tissue and is especially plentiful in aggressive prostate cancers.

Roth's team tested prostate tissue from 338 men with prostate cancer who had their prostates surgically removed between 1995 and 1998.

All of the men had B7-H3 in their prostate tumors. They also had B7-H3 in normal prostate tissue, to a lesser extent. The researchers tracked the men's prostate cancer for up to nine years.

During that time, prostate cancer worsened in 93 of the patients. Those patients generally had higher B7-H3 levels in their prostate tumors than men whose prostate cancer didn't worsen during the study period.