From house dust and toenail clippings to the bond between sisters, health experts hope to unravel some of the mysteries of breast cancer.

It's called the Sister Study, a massive effort to find 50,000 women in the United States and Puerto Rico who have never had breast cancer but who have a sister who has had it. The researchers need to enroll 14,000 more women to complete the study, which is examining how environment and genes affect the risk of developing the potentially deadly disease.

"Sisters share similar genes, but they also had similar early environments and lifestyles and that kind of thing," said Paula Juras, project officer for the study, which is being conducted by the National Institute of Environmental Health Sciences.

"By looking at the sisters, we still are able to look at genes and environmental exposures that may contribute to breast-cancer risk. If you're a woman whose sister had breast cancer, you're twice as likely to have it."

That fact is all-too-familiar to Mary Ellen Strong, a Taylorsville woman who recently enrolled in the study. Her mother and a younger sister have both had breast cancer. Eventually, some of her other sisters may enroll, she said.

"This offered me the opportunity to be proactive in the search for the cause and prevention of breast cancer, and it's so easy to do," said Strong. "I want to do whatever I can do to find a cause. I don't want my granddaughters to face this."

Researchers are seeking women ages 35 to 74 for the study, the largest of its kind ever undertaken. Although the study period extends over 10 years, the time and effort commitment is actually minimal, Juras said.

After a woman enrolls by phone or online, a female health tech will stop by to collect a urine sample, toenail clippings and house dust. The dust can be analyzed for environmental contaminants. The toenails are great at showing exposure to environmental toxic metals. And the urine will answer questions about how the body responds to such exposures.

Researchers say they'll be able to compare women with different exposures and hopefully discover relationships between specific exposures and the cancer.

Sisters share much of their genetic and environmental history. The sister who never had cancer also serves as a sort of control.

"Once a woman is diagnosed with breast cancer, there are many things that may change," Juras said. "Treatment will change things in her body. The disease may change things in her body. She may make changes like diet and exercise. By looking at her sister, we get a better picture of the environment and genes before the disease."