Scientists have pinpointed the cells within a breast cancer that are capable of forming new malignant tumours.
Key cells appear to stimulate cancer growth
They believe that as few as one in 100 cells within a cancer have this capacity.
This may explain why current treatments sometimes fail.
It may also lead to more effective therapies as scientists zero in on the dangerous cells.
If we are to have any real cures in advanced breast cancer, it will be absolutely necessary to eliminate these cells
The key cells, like the body's immature stem cells, have the ability to make copies of themselves, and to produce all the other kinds of cells in the original tumour.
Although similar cells have been identified in human leukaemia, these are the first to be found in solid tumours.
The cells were isolated from breast cancers removed from nine women at the University of Michigan Comprehensive Cancer Center.
They were identified by a specific pattern of proteins on their surface membranes.
They were then injected into laboratory mice where they stimulated the development of new tumours.
Each time cells were taken from a newly grown tumour and injected into new mice they stimulated new cancer growth, and each new tumour was found to have the same cellular make-up as the original sample.
However, many thousands of cancer cells failed to stimulate cancer growth when taken from the original tumour and injected into the animals.
Researcher Dr Max Wicha said the results might explain why some current therapies fail to work.
"The goal of all our existing therapies has been to kill as many cells within the tumour as possible," he said.
"This study suggests that the current model may not be getting us anywhere, because we have been targeting the wrong cells with the wrong treatments.
"Instead, we need to develop drugs targeted at the tumour's stem cells.
"If we are to have any real cures in advanced breast cancer, it will be absolutely necessary to eliminate these cells."
The researchers believe similar cells drive the development of other types of cancer too.
They plan to establish a new research programme to identify stem cells in other cancers and to develop new therapies to destroy them.
Dr Wicha said: "What we are working on now is finding out what makes these tumour stem cells different from the other cells in a tumour.
"Now that we can actually identify them, we can start developing treatments to specifically target and hopefully eliminate them."
Dr Elaine Vickers, of the charity Cancer Research UK, said deaths from breast cancer have fallen by 22% in the last decade, largely due to better treatments and earlier detection.
But she said: "For many women current treatments are unable to halt the spread of the disease, leading to advanced cancer that is much more difficult to treat.
"This research suggests that breast cancers may contain a small number of particularly aggressive cells that determine whether the cancer will spread or not.
"It's an intriguing finding which may ultimately aid the design of new cancer treatments that target these aggressive cells and stop the devastating spread of the disease."
The research is published in the journal Proceedings of the National Academy of Sciences.