The UK’s fertility regulator is about to make a historic decision on whether to allow the creation of babies from three people; a father and two biological mothers.
The technique will help prevent babies being born with deadly genetic diseases.
Three-person IVF has been backed by MPs and peers, got ethical approval and has been shown to be scientifically ready.
A meeting of the Human Fertilisation and Embryology Authority on Thursday will decide whether to give the final go-ahead.
If it does, the first such baby could, at the earliest, be born towards the end of 2017.
How does it work?
The baby would have all the genetic information from its mother and father, plus a tiny amount from a donor woman.
The fertility technique has been developed to prevent deadly mitochondrial disease.
Mitochondria are the tiny structures in every cell that convert food into useable energy.
Defective mitochondria affect one in 200 babies. In severe cases it can leave the child with insufficient energy to keep their heart beating, sustain the brain or move muscles.
Mitochondria are passed on only from the mother, so a second donor egg is needed to create healthy children.
But as mitochondria have their own genetic code, it means resulting children have DNA from three people.
They would have 0.1% of their DNA from the second woman – a permanent change that would be passed down through the generations.
The structure of a cell
Nucleus: where the majority of our DNA is held – this determines how we look and our personality
Mitochondria: often described as the cell’s factories, these convert food to usable energy to make the cell function
Cytoplasm: the jelly-like substance that contains the nucleus and mitochondria.
Is it ethical?
Both the Commons and the Lords approved regulations to allow the creation of such babies in 2015.
And reports by the Nuffield Council of Bioethics and the HFEA concluded the process would be ethical.
But the final safety checks held things up and new concerns emerged that the procedure would fail in one in eight pregnancies.
The HFEA’s science advisors concluded last month that it was time to start.
Prof Robin Lovell Badge, one of the advisors, told the BBC at the time: “We’re not going to learn much more now unless you try it out for real basically – it’s at that stage.
“There’s no reason why it shouldn’t go ahead now, but do it cautiously on selected patients where the risk of having a badly affected child is very high.”
The woman who lost all seven children
Every time Sharon Bernardi became pregnant, she hoped for a healthy child.
But all seven of her children died from a rare genetic disease that affects the central nervous system – three of them just hours after birth.
When her fourth child, Edward, was born, doctors discovered the disease was caused by a defect in Sharon’s mitochondria.
Edward was given drugs and blood transfusions to prevent the lactic acidosis (a kind of blood poisoning) that had killed his siblings.
Five weeks later Sharon and her husband, Neil, were allowed to take Edward to their home in Sunderland for Christmas – but his health slowly began to deteriorate.
Edward survived into adulthood, dying in 2011 at the age of 21.
Now Sharon is supporting medical research that would allow defective mitochondria to be replaced by DNA from another woman.
Has it happened elsewhere?
The UK was the first country in the world to legalise the creation of three-person babies – much of the science has been developed by researchers at the University of Newcastle. .
However, the child will not be the first to be born through the three-person technique.
A Jordanian couple and doctors in New York performed the procedure in Mexico and the resulting baby is understood to be healthy.
Does everyone welcome it?
Robert Meadowcroft, the head of Muscular Dystrophy UK, said Friday’s decision would be “life-changing for many”.
“This is the final move towards clinical trials using an approach to give thousands of women living with devastating and unpredictable mitochondrial conditions the choice to bear their own unaffected children,” he said.
But Dr David King, from the campaign group Human Genetics Alert, warned: “This decision opens the door to the world of GM designer babies.
“Already, bioethicists have started to argue that allowing mitochondrial replacement means that there is no logical basis for resisting GM babies, which is exactly how slippery slopes work.”