When a breast cancer patient has mutations in the best-known susceptibility genes -- BRCA 1 and 2 -- doctors use that information to recommend aggressive treatment.

But, according to new research by scientists at the University of Pennsylvania, for the majority of early onset breast cancer cases involving other mutations, that genetic knowledge will not prove useful to doctors.

Kara Maxwell, an oncologist and lead author of the Penn study being presented in June at the American Society of Clinical Oncology meeting, found that only seven of the 278 screened patients -- or 2.5 percent -- had gene variants that would alter a patient's care plan.

"It's just like we don't do full-body screening for everything, and we don't recommend that people do full-body screening for healthy people," she said of large-panel genetic testing. "This is a lot of information that you can find, and you really need to be thoughtful about the way that you approach it."

For example, Maxwell said, having so many known mutations that contribute to cancer risk complicates the job of genetic counselors, who can advise patients on whether to get tested. But she is nevertheless optimistic.

"It's going to be still slow, but probably about half of the people who we can identify a risk for cancer in their family, I bet you we're gong to be able to figure out how to deal with them faster than we were able to deal with BRCA1 and 2," she said.

The key to that progress, Maxwell said, is for people with family histories of cancer to join research studies.