A woman appears to have been cured of advanced drug-resistant breast cancer after doctors harnessed her own immune system to fight the disease.
Doctors in the US adopted an experimental approach combining two different forms of immunotherapy after conventional hormone treatments and chemotherapy failed.
The outcome was extraordinary, leading to "complete durable regression" of the rapidly spreading cancer that was growing in the patient's liver.
Writing in the journal Nature Medicine, the research team said the unnamed 49-year-old woman had been disease free for almost two years.
British experts described the study as "fascinating and exciting" even though it reported on just one patient.
Professor Alan Melcher, from the Institute of Cancer Research in London, said: "This treatment represents a remarkable success in terms of translating our basic biological understanding of how the immune system responds to cancer into a real treatment of real benefit."
Peter Johnson, Professor of Medical Oncology at Southampton General Hospital, said: "This is another piece of evidence confirming that some cancers are recognisable by the body's immune system and that if this can be stimulated in the right way, even cancers that have spread to different parts of the body may be treatable.
"This particular technique is highly specialised and complex, meaning that it will not be suitable for many people, but it is exciting because it shows how the immune cells already inside cancers may be switched on and made to work better."
The patient had spent 10 years without cancer after having her left breast removed. But then the disease returned with a vengeance and failed to respond to multiple hormonal and chemotherapy treatments.
As well as spreading to the her liver, the recurrent disease also established itself in the woman's right breast.
The US team led by Dr Steven Rosenberg, from the National Cancer Institute in Bethesda, Maryland, identified immune system T-cells within the cancerous breast tissue that were able to recognise and target four mutant tumour proteins.
The immune cells, known as tumour-infiltrating lymphocytes (TILs), were removed from the patient, multiplied under laboratory conditions and injected back into the bloodstream in large numbers.
In addition the scientists deployed one of a range of new immunotherapy drugs called "checkpoint inhibitors" designed to overcome a cancer's ability to shield itself from the immune system.
The antibody drug, pembrolizumab, blocks chemical signals released by some cancers that cause immune system cells to see them as "friendly".
Previously in clinical trials, checkpoint inhibitors have proved ineffective against most breast cancers.
Commenting on the results in the journal, Canadian expert Dr Laszlo Radvanyi, from the Ontario Institute for Cancer Research in Toronto, wrote: "This remarkable response now sets the stage for further testing the effectiveness of neoantigen (tumour protein) specific TIL isolation and therapy on other similar patients with breast cancer."
More importantly, the study helped to dispel "a growing dogma" among cancer specialists that tumours with fewer mutations were not good candidates for targeted immunotherapy treatment, said Dr Radvanyi. These included hormone-driven breast cancer, ovarian cancer and prostate cancer.