Cancer researchers may have underestimated the power of some cancers to spread and cause new tumours, say US researchers.
They found just one skin cancer cell was often enough to generate a whole new tumour.
The finding, published in the journal Nature, undermines hopes that only certain types of cancer cell could fuel the spread of the disease.
UK experts said more work was needed to pinpoint exactly how cancer cells work.
"We think the underestimation of tumour-causing cells is a general problem in many cancers"
The cancer studied by the team from the Howard Hughes Medical Institute and the University of Michigan was melanoma, well known for its ability to spread lethally from a single site.
Normally the ability of a single cell to "seed" a new tumour is tested by injecting large quantities into mice with weakened immune systems and counting how many tumours emerge.
The relatively small proportion of tumours supported the view of many scientists that not all cancer cells could trigger a new tumour, and that this ability was confined to a smaller number of specialist "cancer stem cells".
However, Dr Sean Morrison, who led the latest work, said that this approach was flawed because the mice still had some immunity to these human cancer cells, leading to a significant underestimation of their potency.
One in four
First his team injected melanoma cells into mice with even more severely weakened immune systems, and found that 250,000 times as many of them formed tumours.
When single melanoma cells were used, they discovered that roughly one in four of them went on to seed new tumours.
He said: "As far as we know, this is the first time anyone has been able to show that individual cells from human cancers can efficiently form tumours."
For this reason, identifying and targeting a small subset of these cells just would not work, he said.
"We think the underestimation of tumour-causing cells is a general problem in many cancers, not just specific to melanoma."
He said that researchers needed to make their tests better to see if their cancers were equally potent.
In addition, his team used a battery of tests, but could find nothing marking out any of the cells as potential "cancer stem cells".
While this did not disprove their existence, he said, it could mean that some cancers, such as melanomas, were "good old-fashioned cancer", in which every cell was dangerous.
A spokesman for Cancer Research UK, Ed Yong, said that the idea that tumours grew from a tiny number of "cancer stem cells" was one of the "most interesting" in current research.
"But this study suggests that it may not be true for every type of cancer - in melanoma, a much larger proportion of cancer cells are able to give rise to a new tumour.
"It shows how important it is that we continue to fund research into how cancers develop at a fundamental level."
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