The investigators identified at least 40 genetic alterations that might be attacked by drugs. Many of them are already being developed for other types of cancer that have the same mutations. “We now have a good view of what goes wrong in breast cancer,” said Joe Gray, a genetic expert at Oregon Health & Science University, who was not involved in the study. “We haven’t had that before.”

The study focused on the most common types of breast cancer that are thought to arise in the milk duct. It concentrated on early breast cancers that had not yet spread to other parts of the body in order to find genetic changes that could be attacked, stopping a cancer before it metastasized.

The study’s biggest surprise involved a particularly deadly breast cancer whose tumor cells resemble basal cells of the skin and sweat glands, which are in the deepest layer of the skin. These breast cells form a scaffolding for milk duct cells. This type of cancer is often called triple negative and accounts for a small percentage of breast cancer.

But researchers found that this cancer was entirely different from the other types of breast cancer and much more resembles ovarian cancer and a type of lung cancer.

“It’s incredible,” said Dr. James Ingle of the Mayo Clinic, one of the study’s 348 authors, of the ovarian cancer connection. “It raises the possibility that there may be a common cause.”

There are immediate therapeutic implications. The study gives a biologic reason to try some routine treatments for ovarian cancer instead of a common class of drugs used in breast cancer known as anthracyclines. Anthracyclines, Dr. Ellis said, “are the drugs most breast cancer patients dread because they are associated with heart damage and leukemia.”

A new type of drug, PARP inhibitors, that seems to help squelch ovarian cancers, should also be tried in basal-like breast cancer, Dr. Ellis said.