The new finding offers hints that one type of breast cancer might be vulnerable to drugs that already work against ovarian cancer.
The study, published online Sunday by the journal Nature, is the latest example of research into the biological details of tumors, rather than focusing primarily on where cancer arises in the body.
The hope is that such research can reveal cancer's genetic weaknesses for better drug targeting.
"With this study, we're one giant step closer to understanding the genetic origins of the four major subtypes of breast cancer," Dr. Matthew Ellis of the Washington University School of Medicine said in a statement. He is a co-leader of the research.
"Now we can investigate which drugs work best for patients based on the genetic profiles of their tumors," he said.
The researchers analyzed DNA of breast cancer tumors from 825 patients, looking for abnormalities. Altogether, they reported, breast cancers appear to fall into four main classes when viewed in this way.
One class showed similarities to ovarian cancers, suggesting it may be driven by similar biological developments.
"It's clear they are genetically more similar to ovarian tumors than to other breast cancers," Ellis said. "Whether they can be treated the same way is an intriguing possibility that needs to be explored."
The report is the latest from the Cancer Genome Atlas, a federally funded project that has produced similar analyses for brain, colorectal, lung, and ovarian cancers.