Although the genetic discovery is somewhat alarming, researchers are calling it a good thing; it exposes details about the cancer that should help doctors identify specific drugs to fight it.

The study, published in the Journal of the American Medical Association, is the first to measure how common the different genetic subtypes of breast tumors are in American women, The New York Times reported.

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Among premenopausal black women with breast cancer, 39 percent had the more dangerous kind of tumor, called a basal like subtype, compared with only 14 percent of older black women and 16 percent of non-black women of any age.

The researchers said they are not sure why, the newspaper said.

The study's main author, Dr. Lisa A. Carey -- the medical director of the University of North Carolina-Lineberger breast center -- said the genetic discovery is somewhat alarming.

However, she said it was also a good thing because it exposes details about the cancer that should help doctors identify specific drugs to fight it.