Adult stem cells can pose a cancer risk, warn scientists.
Stem cells are immature "master" cells that can be programmed to become many kinds of tissue and have been heralded as future cures for numerous disease.
Experts have long known that stem cells harvested from embryos can turn cancerous. They have been linked to leukaemias and breast cancer.
Now two research teams have found stem cells from adults can also turn into cancer, reports New Scientist.
Until now, it has been widely assumed that adult stem cells, taken from bone marrow for example, do not form cancers.
A Spanish team at the Autonomous University of Madrid looked at human stem cells that had been extracted from fat tissue.
These stem cells were grown for up to eight months and underwent between 90 and 140 divisions to multiply in number during this time.
When they were transplanted into animals, the oldest stem cells formed cancers, the study authors reported in the latest edition of the journal Cancer Research.
In the same journal, a team of Danish researchers from the Institute of Cancer Biology suggest how these stem cells become cancerous.
They believe it is to do with cells replicating too many times.
They found that the cells that became cancerous had started making an enzyme, called telomerase, that enables them to continue dividing long after they would normally stop.
The Madrid team said that the risk was mainly theoretical, because the stem cells treatments being tested in humans at the moment used cells that were only briefly grown outside the body.
However, treatments often require vast numbers of stem cells that have to be grown from very small numbers.
Also, stem cells stored for years in banks might not be safe, they said.
Antonio Bernad and colleagues suggest that the cut-off point should be around 60 divisions or generations, but recommended more research.
Professor Christopher Higgins, of the Medical Research Council Clinical Science Centre, said: "Stem cell lines maintained and developed in stem cell banks over a long period of time are currently only used for research purposes. They are not transplanted into people.
"Stem cells can potentially be of enormous benefit for clinical treatment but as with the development of all drugs and therapies, there are safety issues that need to be investigated and resolved.
"The MRC is conducting extensive research to help understand the risks involved with stem cell development and to develop safe and effective therapies."
Dr Kat Arney from Cancer Research UK said: "Research like this is extremely important to help build up a full picture and develop the best possible treatments.
"Many patients suffering from serious diseases, including cancer, could potentially benefit from stem cell research."