A patient has seen his advanced skin cancer disappear after doctors treated him with clones of his own immune cells.
US scientists replicated thousands of the body's cells that fight cancer and infection, and infused them back into the patient. Over a period of two months, scans showed the 52-year-old man's tumours were completely gone.
It is the first solid evidence that the experimental T-cell treatment works.
But the US scientists are quick to point out that the breakthrough involved just one patient.
Advanced malignant melanoma, the most dangerous form of skin cancer, is notoriously difficult to treat once it starts to spread.
The US team, led by Dr Cassian Yee, from the Fred Hutchinson Cancer Research Centre in Seattle, pioneered a new therapy based on infection-fighting "helper" CD4 T-cells from the patient's own immune system.
Helper T-cells are specialised white blood cells that "target" foreign invaders or cancerous cells, and marshal other elements of the immune system against them.
The new technique involved extracting helper T-cells from the patient and cloning those specifically targeting melanoma. They were then stimulated to divide and multiply in the laboratory, boosting their numbers 3,000 to 5,000-fold.
Before treatment, the man had Stage 4 advanced melanoma which had spread to a groin lymph node and one of his lungs.