By Ania Lichtarowicz
BBC health correspondent
Researchers in the US say they have been using stem cells to successfully treat genetic diseases in children.
Blood taken at birth from the umbilical cord is rich in stem cells
The doctors told a conference that cells taken from umbilical cord blood can turn into healthy heart cells, for instance, and repair damaged tissue.
The research suggests many childhood disorders such as leukaemia can be treated in this method.
For scientists, this confirms what laboratory tests have long suggested - stem cell treatment works in the body.
Researchers at Duke University have been using cord blood to treat children with rare diseases affecting the heart, liver and brain, but until now have been unsure as to why their treatment was successful.
Umbilical cord blood contains a small amount of stem cells, which many scientists believe could be used to grow into new healthy tissue.
Many scientists have already done this in the lab, but this is the first time they have found evidence that it is happening in the body.
They discovered that the stem cells can enter damaged heart tissue and transform themselves into healthy heart cells, so preventing further damage.
As stem cell research develops, scientists are seeing more and more potential in using stem cells collected at birth.
The National Health Service in the UK has recently invested large amounts of money into storing cord blood from newborn babies, and a number of private companies in the US and Europe are also offering cord blood storage services.
These stem cells would work best in the child that they came from.
However, because cord blood is very clean, the cells could also be effective in other children - and one day, even adults.