A 'designer baby', has been confirmed as a perfect genetic match with the brother his stem cells could save.
Jamie Whitaker and his older brother Charlie
Baby Jamie Whitaker was born in June after his embryo was genetically selected in the hope that it would be a match for his four-year-old brother Charlie.
Charlie has the life-threatening blood disorder Diamond Blackfan Anaemia, and a stem cell transplant is his only hope of a cure.
Blood tests have now shown that month-old Jamie is a match, so stem cells from his umbilical cord can be used to treat Charlie.
The boys' parents, Michelle and Jayson Whitaker, say they are delighted by the news.
The family, from Derbyshire, must now wait until Jamie is around six months old to ensure he does not have the blood disorder before a stem cell transplant can go ahead.
Jamie was born by Caesarean section at the Jessop Wing of Sheffield's Royal Hallamshire Hospital.
Michelle Whitaker told the BBC: "Next year's going to be quite hard for Charlie.
"But hopefully this time next year, it will be over and done with.
"He'll have had chemotherapy, his stem cell transplant, and it will all be done and dusted and he'll just be a normal healthy child."
Dr Mohamed Taranissi, the UK doctor who has been treating the Whitakers, told BBC News Online: "We know now that Jamie is an exact genetic match.
"And we know which embryo gave us the new baby, which tells us how accurate the technique is."
Dr Taranissi said he had been fairly confident the baby would be a match during the pregnancy.
But he added: "We always tell people there is a 98% chance of accuracy.
"But there was no point carrying out a test during pregnancy in this case because even if it was in the 2% that was not an exact match, they would still have kept the pregnancy.
"That shows that this baby was not just created for one reason."
The couple had to travel to America for embryo screening after being denied permission by fertility authorities for it to go ahead in the UK.
The Human Fertilisation and Embryology Authority said it was acceptable to test and select embryos to prevent the birth of a baby with a genetic disease, but not to select them in order to help another child.
Charlie was the second UK baby born to help cure a sibling.
A couple whose child was suffering from leukaemia and needed a bone marrow transplant underwent the same the same route in 2001.