Detectives using DNA to hunt criminals should be aware bone marrow transplants can confuse results, say scientists.
Marrow recipients can have a mix of their own and the donor's DNA
New Scientist reports on an Alaska case in which a man was linked to an attack, based on DNA obtained with blood tests, but had been in jail at the time.
It transpired the sex attack had been carried out by his brother, who had donated bone marrow to his sibling in a transplant some years earlier.
The same risk would not apply in the UK where DNA is checked via cheek swabs.
Miscarriages of justice
Abirami Chidambaram, of the Alaska State Scientific Crime Detection Laboratory, in Anchorage, said the case highlighted the danger of miscarriages of justice.
When she first found the DNA match between the semen from the sexual assault and the jailed man's blood, she thought there must have been some kind of mix-up with the samples, she told a meeting of the American Society of Human Genetics, in Salt Lake City
However, further investigation revealed the real culprit was the jailed man's brother.
The bone marrow transplant meant that the jailed man's blood contained cells bearing his brother's DNA.
New Scientist said: "As forensic DNA databases grow and more people undergo bone marrow transplants, the risk of a miscarriage of justice increases."
David Lazer, of Harvard University and an expert on the use of DNA in the US criminal justice system, said: "It makes sense for investigators to be savvy to this."
He said he hoped it would not put potential bone marrow donors off donating.
Similar confusion should not arise so readily in the UK because of the samples used to test for a DNA match. The UK uses cheek swabs rather than blood samples.
Donors urgently needed
Dr Sue Pope, of the Forensic Science Service, said research suggested that such samples from transplant recipients would contain a mixture of the person's own DNA and their donor's DNA.
"It would show up so it could not go into the national DNA database anyway."
But she said it might be sensible for people who have donated bone marrow to volunteer this information should any question of DNA identity arise.
Dr Paul Travers, from The Anthony Nolan Trust, said the actual risk to any donor of mistaken identity was negligible.
The trust says it urgently needs more donors to save lives.
At any one time, there are about 7,000 patients throughout the world needing to find a suitable bone marrow donor.
Over 4,000 patients have been given life-saving treatment thanks to The Anthony Nolan Trust's register.