The Sister Study, conducted by the National Institute of Environmental Health Sciences, part of the US government's National Institutes of Health, is the largest trial 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 on 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. 

Other factors

"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." 

"Genes are important, but they don't explain it all"

Dr Dale Sandler,
the study's principal investigator

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% to 10% of breast cancer cases. 

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.