MONDAY, May 1 (HealthDay News) -- U.S. researchers say they've discovered a biological marker to identify breast cancer survivors at risk for long-term persistent fatigue.

Nearly a third of breast cancer survivors experience disabling fatigue that lasts for years. The fatigue is caused by responses within their immune systems.

"These studies identify a biological basis for persistent fatigue in cancer survivors that is implemented by inflammation," Dr. Michael Irwin, director and senior research scientist of the Cousins Center for Psychoneuroimmunology at the University of California, Los Angeles' Semel Institute for Neuroscience, said in a prepared statement.

"We have detected a biological marker that is a composite of two immune response elements. This biomarker identifies -- and can predict -- which women have long-term persistent fatigue," Irwin said.

"These findings point the way for development of novel treatment strategies that decrease this inflammatory response and thwart the fatigue that these patients endure," he added.

One component of the marker is to measure the amount of interleukin-6 receptor (IL-6R) free-floating in the blood. The other component of the marker is an index measured by the level of T-cells that are characterized by CD69, a cell membrane protein that indicates early activation of T-cells.

The findings appear in the May 1 issue of Clinical Cancer Research.