Around 10,000 women a year in the UK might benefit from a new type of breast cancer treatment, say scientists.
Biological therapies can help fight breast cancers caused by rare, inherited genetic errors like the BRCA one actress Angelina Jolie carries.
Now a new study by experts at the Wellcome Trust Sanger Institute suggests these targeted drugs may also work in many other women who do not have these risky genes.
The drugs could be effective in one in five breast cancers, say the researchers.
That's 20% of patients - far more than the 1 to 5% who develop the cancer alongside having faulty BRCA genes.
One biological therapy or PARP inhibitor, called olaparib, is already used on the NHS to treat advanced ovarian cancer.
It is not yet approved as a breast cancer drug, although some UK women are taking it in clinical trials.
For the latest work, published in the journal Nature Medicine, the researchers looked at the genetic make-up of breast cancer in 560 different patients.
They found a significant proportion of them had genetic errors or "mutational signatures" that were very similar to faulty BRCA.
Given the close similarity, these cancers might be treatable with biological therapies too, they reasoned.
They recommended clinical trials to confirm this.
Baroness Delyth Morgan, from Breast Cancer Now, called the early results "a revelation".
"We hope it could now lead to a watershed moment for the use of mutational signatures in treating the disease," she said.
One of the researchers, Dr Helen Davies, said there was also the potential to treat other types of cancers with these drugs.
Biological therapies have already had some promising results for treating prostate cancer.
They change the way cells work and help the body control the growth of cancer.
Carrying certain gene mutations, like faulty BRCA, increases a woman's risk of developing breast cancer, although it does not mean she will definitely go on to get cancer.
Some women - like Angelina Jolie - opt to have surgery to have their breasts removed to lower their lifetime risk.
Vicki Gilbert, 54 and from Swindon, found out she was carrying high-risk genes - but only after she developed breast cancer.
"I had been thinking about getting tested anyway because there was quite a lot of cancer on one side of my family. But then I was diagnosed with breast cancer out of the blue, before I even had a chance to go for the genetic screening."
She says finding out, even after the event, was helpful.
"When you get cancer you do think 'Why me?' I don't know for sure if it was because of the genes that I inherited. That would be impossible to say. But, for practical reasons, it is useful to know that I carry these genes."
Vicki has been free of cancer for around seven years, but still has regular checks because of her increased genetic risk.
Women can lower their lifetime risk of breast cancer by exercising regularly, eating a good diet, maintaining a healthy weight, avoiding cigarettes and limiting how much alcohol they drink.
Michelle Roberts - BBC Health News