US scientists hope to be able to use a harmless form of the Aids virus to seek and destroy cancer cells.
The virus targeted cancer cells
A University of California team found an "impotent" version of HIV, with the disease-causing parts of it removed, tracked down cancer cells in mice.
The next step would be to insert a gene into the virus that would kill the cancer upon contact.
The team told Nature Medicine more safety studies were needed before such a method could be tested in humans.
The mice they studied had a form of skin cancer, called melanoma, that had spread to the lungs.
In the laboratory, the scientists took HIV and removed the parts of the virus that causes disease.
They then stripped off the virus' outer coat and redressed it with the outer suit of another virus.
By doing this, the researchers had changed the target of the virus.
HIV normally infects immune cells called T cells. The new outer coat instead directed HIV to hunt down molecules present on cancer cells, called P-glycoproteins.
The scientists also added a substance to the virus that would make it visibly glow when looked at with a special camera so they could track where it travelled once injected into the mice.
Researcher Dr Irvin Chen, from UCLA's Aids Institute, said: "The virus travelled through the bloodstream and homed straight to the cancer cells in the lungs, where the melanoma had migrated.
"Gene therapy has been hampered by the lack of a good carrier.
"Our approach proves that it is possible to develop an effective carrier and reprogram it to target specific cells in the body."
Beating cancer's spread
His team is planning to see whether the virus could carry a therapeutic gene to the precise location of the cancer.
As well as controlling cancer, they hope this technique might be useful for treating genetic diseases.
Dr Georges Vassaux, from Cancer Research UK's clinical centre at Barts and The London, said: "This is the first time that a vector - or delivery system - for gene therapy has targeted a tumour in such a specific manner.
"This means the technique could be used to use gene therapy in cases where cancer has spread around the body.
"So far gene therapy has been successfully used only on tumours that are confined to their original location."
He said there had been concerns that such methods might cause leukaemia in normal cells.
"As the team has managed to target the therapy to cancer cells, it looks as though a hazard associated with the use of integrative viruses may have been overcome," he said.