updated 10/18/2004 2:57:30 PM ET 2004-10-18T18:57:30

A new national study will investigate genetic and environmental causes of breast cancer by enrolling 50,000 sisters of women already diagnosed with the disease.

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The Sister Study, conducted by the National Institute of Environmental Health Sciences, part of the government’s National Institutes of Health, is the largest study of its kind.

“By studying sisters who share the same genes, often had similar experiences and environments, and are at twice the risk of developing breast cancer, we have a better chance of learning what causes this disease,” Dr. Dale Sandler, the study’s principal investigator, said in a statement Monday.

The sisters who volunteer will donate blood, urine, toenails — even household dust — to help uncover how daily rituals and routines, as well as genetics, factor into breast cancer risk.

“Genes are important, but they don’t explain it all,” said Sandler, chief of the epidemiology branch at the environmental health institute. “The truth is that only half of breast cancer cases can be attributed to known factors.”

For instance, BRCA1 and BRCA2 are genes that normally limit cell growth. Women who inherit an altered version of either gene have a higher risk of getting breast or ovarian cancer. BRCA1 and BRCA2, however, are implicated in just 5 percent to 10 percent of breast cancer cases.

Tracked for 10 years
Breast cancer is the second most commonly diagnosed cancer in women, after skin cancer. Some 215,990 American women will be diagnosed with breast cancer this year, according to the American Cancer Society. The disease will kill about 40,110 American women in 2004, according to the Centers for Disease Control and Prevention.

To be eligible for the study, women need to be between 35 and 74 years old. Women who have not been diagnosed with breast cancer are eligible if a sister, living or dead, has had breast cancer. The women will be tracked for 10 years so researchers can study what links the few who get breast cancer compared with the majority who do not.

The study began as a pilot in Arizona, Florida, Illinois, Missouri, North Carolina, Ohio, Rhode Island and Virginia.

Cruz Mireles, of Peoria, Ariz., joined after seeing a booth with study details at Komen Race for the Cure, an event she’s done annually since her younger sister, Olivia Hernandez, was diagnosed with breast cancer at age 40. Mireles is one of seven girls in the family. She and two sisters who live in Arizona already participate in the Sister Study.

“I would like to see breast cancer eradicated, hopefully in my daughter’s lifetime,” said Mireles, 58, referring to her 34-year-old daughter. “It was very important to me that we try to reach a solution.”

Dottie Sterling, a volunteer in Ohio, joined when asked by her youngest sister, Wish Martin, a breast cancer survivor in Maryland.

“My sister has been a breast cancer survivor for more than 13 years and I could not be more proud,” Sterling said in a statement. “I see joining the Sister Study as my tribute to her strength and her faith.”

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