Scientists believe they have found a new way to fight breast cancers that run in families.
Most tumours are not linked to inherited genes
The experimental drug fights tumours linked to two faulty genes, BRCA1 and BRCA2, which are known to increase a woman's breast cancer risk by 50-80%.
The Institute of Cancer Research team believes the drug, a "PARP-inhibitor", will be less toxic to healthy cells than standard chemotherapy.
The study, funded by charities, is described in the journal Nature.
In the UK, nearly 41,000 women are diagnosed with breast cancer each year.
Of these, about 5% are due to strong hereditary factors, some of which are caused by BRCA1 and BRCA2 mutations.
Professor Alan Ashworth and colleagues from the Breakthrough Breast Cancer Research Centre, based at the Institute of Cancer Research, designed a drug that would specifically target tumour cells but leave normal cells alone.
This should avoid unwanted chemotherapy side effects such as hair loss and nausea.
Breast cancers in women who inherit faults in either the BRCA1 or BRCA2 genes occur because the tumour cells have lost a specific mechanism that repairs DNA.
What PARP-inhibitors do is seek out and kill any cells that have lost the DNA repair mechanism.
Professor Ashworth explained: "This is a brand new therapeutic approach, centred on exploiting a specific deficiency in breast cancer cells - their Achilles' heel.
"And because it is targeted, it should be much less toxic."
So far the drug has only been tested in the laboratory. Professor Ashworth said larger safety trials would begin in June.
If these are successful they will then test the treatment on patients with BRCA-linked tumours.
"We hope that it will be available to women in the next few years," said Professor Ashworth.
He said PARP-inhibitors might also be good for treating other forms of breast cancer that behave in a similar way, which account for a fifth of all breast tumours.
The work, in collaboration with KuDOS Pharmaceuticals, was funded by Breakthrough Breast Cancer, Cancer Research UK, The Wellcome Trust and the Mary-Jean Mitchell Green Foundation.
Jackie Harris from Breast Cancer Care said: "We are excited to hear about this new drug, a PARP inhibitor, which targets specific tumour types.
"This is great news for women who carry the gene faults BRCA 1 and BRCA 2.
"However, as the drug has only been tested in the laboratory, we look forward to hearing the outcome of the Phase 1 trials in the near future."