Epidemiologists have known for a while that Latina women have a lower risk for breast cancer than white or black women, but they never knew why. Some doctors figured it must have something to do with European or African genes that make them more susceptible.

But in fact, it's the variation of a single gene in chromosome six of many people of indigenous American ancestry that acts like a shield against breast cancer. "The effect is quite significant," said senior author of a new study, Dr. Elad Ziv, of the University of California, San Diego, in a news release. "If you have one copy of this variant, which is the case for approximately 20 percent ... of U.S. Latinas, you are about 40 percent less likely to have breast cancer." A small minority of women who have two copies of this gene are 80 percent less likely to get breast cancer.

The study, published Monday in the journal Nature Communications, looked at more than 11,000 women of different ethnicities. They ran an analysis of their entire genomes, looking for small differences in their DNA that could affect the likelihood of a breast cancer diagnosis. Finally, they found it, in a single piece of genetic coding for an estrogen receptor.

A version of this gene in indigenous Americans seems to interfere with the development of the most aggressive types of breast cancer, a form known as estrogen receptor–negative. The lifetime risk of breast cancer for Hispanic women is 10 percent. It's 11 percent for blacks and 13 percent for whites, according to data from the National Cancer Institute.

Now that the doctors know about the gene, they might be able to home in on it for therapy or to create more accurate methods of breast cancer screening. Those plans are already in the works, they said. "If we can use these results to better understand how this protects estrogen receptor-negative breast cancer, that would be interesting and important," Ziv said. "Right now we have no good way to prevent that type of breast cancer."

Source: E. Ziv, et al. Nature Communications. 2014.