BBC health reporter
Scientists in the United States have warned that some forms of gene therapy may cause patients to develop cancer.
Insulin-making genes were injected into the mice
Writing on the journal Nature Genetics, the researchers have found that genes inserted into cells to replace faulty copies may be spreading and damagining healthy genes.
The news comes after a number of children developed leukaemia following gene therapy treatment.
High profile gene therapy trials have been stopped in France and but are continuing in the UK despite a number of children developing leukaemia follwing treatment.
Originally the trials were considered a huge success when a number of patients were successfully treated for the X-linked Severe Combined Immunodeficiency Syndrome (X-Scid) which is more commonly known as the "Boy in the Bubble" syndrome.
Boys suffering from the condition can't make normal white blood cells, which fight off infection.
As a result, they usually die by the age of two.
This latest research - which was carried out in mice - supports the theory that the cancer in these patients was caused by the new treatment.
Scientists found that after injecting genes into the livers of mice, in some cases, the new genes became incorporated into the cells chromosomes.
On taking a closer look they found that the new genes placed themselves inside normal genes and so could potentially damage their functioning or could even cause cancer.
If this does turn out to be the case, scientists may well have to develop a safer way of delivering new genes into the body, if the enormous promise of gene therapy is ever to become reality.