The U.K. has become the first country to approve a controversial fertility treatment that involves creating babies who have three biological parents. The move is expected to help some women with rare genetic defects become mothers as soon as late 2017.

The U.K. government’s Human Fertilisation and Embryo Authority has said that clinics around the country are now allowed to apply for licenses to perform mitochondrial replacement therapy. The technique involves taking a nucleus from an egg or fertilized embryo that has defective mitochondria, and swapping it into a healthy donor egg.

One clinic, the Newcastle Fertility Centre, said it planned to apply for a license within 24 hours of the announcement. A team at Newcastle University said it hopes to treat up to 25 patients a year using the technique.

Mitochondria are the powerhouses of cells. And while they contain just a tiny amount of genetic information, rare mutations in the mitochondria in a mother’s egg (men don’t pass down mitochondria to their children) can cause children to be born with cells that don’t produce enough energy. The condition is often fatal.

Doctors have worked for years to find treatments, and three-parent babies were born in the late 1990s and early 2000s using a different technique. But fears that the method, known as cytoplasmic transfer, caused birth defects led the Food and Drug Administration to ask clinics to stop the procedure. More recently, Congress has made it clear it isn’t interested in allowing embryos to be genetically modified, even if it means avoiding an awful disease.  

An embryo that results from swapping a nucleus into a donor egg will contain genetic information from two women and one man—giving rise to a genetically modified person. But the donor’s DNA will only be present in the form of mitochondria, which don’t play a role in traits like a person’s looks or personality.

Earlier this year, a baby boy was born in this way. And though he was born in the U.S., the procedure for creating him was carried out in Mexico, where no laws govern genetically modified embryos. In giving the technique formal approval, authorities in the U.K. say, they will be able to ensure procedures are tightly regulated and children born using mitochondrial replacement are monitored to ensure that they develop normally.

(Read more: The Guardian, New Scientist, “A Three-Parent Child Was Conceived in Mexico, Because the U.S. Won’t Allow It,” “The Unintended Consequence of Congress’s Ban on Designer Babies”)