For the first time, a baby has been born using the DNA from three people in the United Kingdom.
It has recently become easier and safer to do after scientists further developed a biotechnological technique called 'mitochondrial donation'.
Director of the Progress Educational Trust in London, Sarah Norcross, said it'll be a slow process to expand the technology's use.
"News that a small number of babies with donated mitochondria have now been born in the UK is the next step in what will probably remain a slow and cautious process of assessing and refining mitochondrial donation," she said.
But the UK wasn't actually the first time - a Jordanian family getting treatment in the US had a healthy baby in 2016 using the same technique.
SO WHAT ARE 'MITOCHONDRIA'?
Mitochondria are described as the 'powerhouse' units and they're in almost every cell in your body.
Trillions of these tiny units convert the food we eat into usable energy through chemical reactions - but if they're defective, or there aren't enough, the body can't get enough fuel.
This leads to mitochondrial diseases - including brain damage, heart failure, blindness, or wasting of muscles.
Mitochondria also have their own DNA, so changes to their genetic makeup will be passed on to future generations.
THEN WHAT IS 'MITOCHONDRIAL DONATION'?
In this case, most of the baby's DNA (genetic instructions) came from its two parents, but 0.1 percent came from a third, donor woman's mitochondria.
'Mitochondrial donation' refers to replacing the faulty mitochondria in the mother's egg with healthy mitochondria from a donor.
Traits like appearance or personality are not affected, and babies born with the technique do not have a 'third parent'.
WHY IS IT NECESSARY?
Scientists hope to prevent children being born with mitochondrial diseases, which can shorten their lives to days or even hours of birth.
Families throughout the world have lost several children to these diseases, and mitochondrial donation is seen as the only way to have a healthy child.
Professor Robin Lovell-Badge from the Francis Crick Research Institute would like to see if these babies are truly spared from mitochondrial diseases for their entire lifespans.
"It will be interesting to know how well the mitochondrial replacement therapy technique worked at a practical level, whether the babies are free of mitochondrial disease, and whether there is any risk of them developing problems later in life."
WHERE DID THIS TECHNOLOGY COME FROM?
It originated as a special form of IVF (in-vitro fertilisation) to allow mothers to have babies with healthy mitochondria.
The first baby using this procedure was born in 1997 in the US.
But unlike IVF, instead of using a donor's whole egg, only the mitochondria are needed.
Lawmakers in the UK passed a law in 2015 to allow these children to be born, paving the way for the techniques to be developed in Newcastle.
The UK's fertility authority, called the HFEA (Human Fertilisation and Embryology Authority), said fewer than five babies worldwide have been born using this technology as of April.
NEXT STEPS
The risk of "reversion" remains, where any non-functional mitochondria could still be passed on to the baby, later multiplying, and resulting in disease.
But there's still hope, and with biotechnology's scope expanding at a break-neck speed could see hundreds of these babies born in the UK each year.
WHAT IS THE PROCESS?
Method 1: Using an egg repaired AFTER fertilisation
Method 2: Using an egg repaired BEFORE fertilisation
