Stem cells could be used to treat a killer lung disease, say experts.
Cystic fibrosis clogs the lungs with thick, sticky mucus
A team at London's Great Ormond Street Hospital points to evidence in two young boys with cystic fibrosis who had been given lung transplants.
The donor lungs had come from girls, yet some of the boys' new lung cells contained the male Y chromosome, suggesting they were self-made.
The Thorax study could help settle the debate about whether stem cells become lung cells and are useful as therapy.
Doctors have shown that people who have had bone marrow transplants develop lung cells that resemble the genetic make up of the donor rather than themselves.
But others doubt whether stem cells - very primitive cells that have yet to develop fully - can become mature lung cells.
If they can, stem cells could be used to treat and ultimately cure cystic fibrosis, which is caused by a faulty gene.
More than 7,500 people - about half of whom are children - have CF in the UK.
The average life expectancy for patients is 31 years. Most die from complications of the lung which is the main organ affected.
The authors of the latest study, funded by The Cystic Fibrosis Trust, argue their findings show the patients' own stem cells must have travelled in the blood to the lung and turned into new lung cells.
Dr Adam Jaffe's team looked at lung tissue samples taken from the boys for more than a year after the lung transplant operation.
Dr Jaffe said: "It's extremely exciting. This really is a true phenomenon.
"It raises an avenue to explore in the ultimate aim of curing CF, although that would be a long way off.
"The ultimate dream would be to harvest these stem cells of patients with CF, put a normal copy of the defected gene into these cells, and then put the cells back into the patient.
"This may then help to prevent the damage caused by the disease.
"The same has been done successfully in children with a different genetic disease that affects the bones."
He said the next step was to find out where the stem cells come from.
"My hunch is that they come from the bone marrow," he said.
He said they will also need to find out what encourages these cells to grow.
Professor Julia Polak, director of tissue engineering and regenerative medicine at Imperial College, said: "We have our own evidence showing that these cells do come from the bone marrow.
"It's a bit early to say, but it might be possible one day to mobilise these cells as a therapy."
She said another potential route was to grow lung cells from donor stem cells in the laboratory.
"We do not know what will be the best course to help patients yet."
Dr Steven Hyde, from the Gene Medicine Research Group at the University of Oxford, said researchers were also looking at delivering gene therapy direct to the lungs with an aerosol spray.
"It's a highly contentious area of research. We don't have the answers yet - the jury is still out."
"But they are really interesting early findings."
Rosie Barnes, chief executive of the Cystic Fibrosis Trust, said the findings would help experts working to identify and correct the defective gene that causes CF.