By Fergus Walsh
BBC News science correspondent
British doctors say they have made a significant advance in the fight against genetic disease.
Charlie is now full of energy
They believe they have cured a six-year-old boy of a rare blood disorder after transplanting cells from his baby brother who was created to save him.
Doctors say the technique could be used to help many other children with blood and metabolic disorders.
But critics say creating a baby in order to treat a sick sibling raises ethical concerns.
Charlie Whitaker, from Derbyshire, was born with Diamond Blackfan Anaemia, a condition which prevented him from creating his own red blood cells.
He needed blood transfusions every three weeks, and drug infusions nearly every night.
But his condition has been cured by a transplant of cells from the umbilical cord of his baby brother Jamie.
The case is controversial because Jamie was genetically selected to be a donor.
His parents' embryos were screened to find one which was a perfect tissue match.
The couple were refused permission for the treatment so they travelled to Chicago to have it.
Neither of Charlie's parents, Michelle and Jayson, or his younger sister, Emily, were a suitable match.
Charlie with his baby brother
Dr Ajay Vora is the consultant haematologist at Sheffield Children's Hospital who is looking after Charlie.
He said tests on his bone marrow three months on from his transplant had produced very promising results.
He said: "I'm pleased to say his bone marrow looks entirely normal, and I would go so far as to say he is effectively cured of his Diamond Blackfan Anaemia.
"We still need longer follow-up to be 100% certain of that, but so far what we have seen looks to be very, very positive.
"The staff are extremely pleased that Charlie's transplant has been relatively uncomplicated, and that he can look forward to a normal quality of life."
Charlie's mother Michelle says looking at Charlie now its all been worthwhile.
"Charlie's energy level has changed dramatically. He is on the go constantly - he is just like a different person. It feels wonderful."
Charlie's parents are delighted
The Human Fertilisation and Embryology Authority first said that it was unethical to create a life simply to save another life, then changed its decision earlier this year.
John Harris, Professor of Ethics at Manchester University says this is not a slippery slope towards designer or spare parts babies.
"What better reason could there conceivably be for having a child than to save the life of an existing person? There is no better reason," he said.
"The HFEA are to be congratulated on changing their mind. But one must remember that their original decision, if it had stood and if the Whitaker's had taken notice of it, would have resulted in one very sick child - possibly a dead child, and now there are two healthy, happy children."
However, the group Comment on Reproductive Ethics has called the process of screening embryos to create a child to serve as a tissue donor for a sick sibling "undesirable and unnecessary".
David King, from Human Genetics Alert, said: "The main problem with using this technology is that it is basically creating a child not for its own end, but for the purposes of being a donor.
"So you are using that child as a means to an end, rather than as an end in itself."
Three other couples are now going through the same treatment in Britain and doctors estimate that up to 100 each year could be treated by genetically selected donor babies.