Fertility doctors using a controversial technique developed to eliminate inherited disease have helped a woman deliver the first baby with three genetic parents.
Five-month-old Abrahim Hassan's Jordanian mother was treated by a US team in Mexico.
He was conceived from an egg containing nuclear DNA from his mother and father, and mitochondrial DNA from a "second" mother - an unknown female donor.
The aim was to replace defective mitochondrial DNA (mtDNA) that may have condemned Abrahim to Leigh syndrome, a fatal disorder affecting the developing nervous system.
Mitochondrial DNA is housed in the tiny rod-like "batteries" in cells that supply energy, and is passed down through the generations by mothers.
So far only the UK has given an official stamp of legal approval to the technique employed by the American doctors.
But the US team moved ahead by achieving the first live birth involving mitochondrial replacement.
Dr Dusko Ilic, an expert from King's College London, told New Scientist magazine, which carries an exclusive report of the birth in its latest issue: "This is great news and a huge deal. It's revolutionary".
Inherited mitochondrial diseases include devastating conditions that result in poor growth, muscle weakness, loss of co-ordination, seizures, vision and hearing problems, learning disabilities and organ failure.
It is estimated that one in 4,000 people has an incurable mitochondrial disease.
Abrahim's mother, Ibtisam Shaban, carries the Leigh syndrome mutation in about a quarter of her mitochondria.
While she has suffered no ill effects, the disease killed her first two children, said New Scientist.
Ms Shaban and her husband, Mahmoud Hassan, sought the help of leading embryologist Dr John Zhang, medical director at the New Hope Fertility Centre in New York City.
He used an approach called spindle nuclear transfer, which involves finding an egg from a willing donor and removing its nuclear DNA.
The nucleus from one of Ms Shaban's eggs was then inserted into the donor egg.
The result - an egg containing nuclear DNA from Ms Shaban and mtDNA from the donor - was fertilised with Mr Hassan's sperm.
Five embryos were created, only one of which developed normally and was implanted in Ms Shaban's womb.
While the US has not approved the technique, Mexico has "no rules", Dr Zhang is reported to have said.
He told New Scientist: "To save lives is the ethical thing to do."
Tests of Abrahim's mitochondria showed that less than 1% carried the Leigh mutation, which is thought to be far too low to cause any problems.
The work was briefly reported in a little-known scientific journal, Fertility and Sterility, in an article entitled "First live birth using human oocytes reconstituted by spindle nuclear transfer for mitochondrial DNA mutation causing Leigh syndrome".
British experts congratulated the team but called for more information.
Professor Doug Turnbull, from the University of Newcastle, said: "There have been extensive discussions in the UK to ensure that families with mitochondrial disease get the best possible advice about their reproductive options and that any new IVF-based technique is appropriately regulated and funded.
"This abstract gives very little information about the technique used, the follow-up of the child or the ethical approval process."
Dr Ilic added: "This was an ice-breaker. The baby is reportedly healthy; hopefully, this will tame the more zealous critics, accelerate the field, and we will witness soon a birth of the first mitochondrial donation baby in the UK.
"But some questions remain. By performing the treatment in Mexico, the team were not subject to the same stringent regulation as some other countries would insist on. We have no way of knowing how skilful or prepared they were, and this may have been a risky thing to do. On the other hand, we have what appears to be a healthy baby. Because it was successful, fewer questions will be raised but it is important that we still ask them."
Although Fertility and Sterility released a short summary of the procedure, full details have not yet been reported.
Dr Ilic said: "Was this the first time ever they performed the technique or were there other attempts and they are reporting this one because it was successful? This and other important questions remain unanswered because this work has not been published and the rest of the scientific community has been unable to examine it in detail. It's vital that that happens soon."
Professor Simon Fishel, president of the Care Fertility Group of IVF clinics, said: "This is a devastating disease that we do not wish children to be born with, and what's more, we would wish to eradicate from any family lineage - and this technology will help achieve this. However, any first-time medical procedure moving from successful animal studies to the human, that will take several years until we understand its impact, is a necessary experimental step forward, which we hope and believe should be totally safe and utterly effective.
"Congratulations to this team and all those hoping to help these unfortunate families, and we should proceed with caution and due regulation."
Last year the House of Lords approved legislation to allow IVF involving mitochondrial replacement, making the UK the first country in the world explicitly to approve the procedure.
Strict controls remain in force in the UK and scientists wishing to follow in Dr Zhang's footsteps would first have to obtain permission from the Human Fertilisation and Embryology Authority.
Dr David King, director of the watchdog group Human Genetics Alert, branded the research "unethical and irresponsible".
He added: "It is outrageous that they simply ignored the cautious approach of US regulators and went to Mexico, because they think they know better.
"Since when is a simplistic 'to save lives is the ethical thing to do' a balanced medical ethics approach, especially when no lives were being saved?
"These scientists have used an experimental technique that many scientists still think is unsafe in order to create a world first.
"When are the world's governments going to stop rogue scientists crossing crucial ethical lines?"