Researchers at the University of Texas Southwestern Medical Center in Dallas say they believe estrogen may deactivate genes in breast tissue that suppress breast-cancer development.

Dr. Yvonne Coyle, a physician who researches the epidemiology of cancer, said when estrogen silences these so-called tumor-suppressor genes by a process called methylation -- which is reversible -- the risk for breast cancer increases.

"So many of the risk factors for breast cancer cannot be modified, such as being a woman, getting older or having a family history," Coyle said in a statement. "However, physical activity may be a modifiable risk factor allowing for breast cancer risk reduction."