Techniques for using genetic material from three people to make embryos are designed to prevent mothers with defects in their mitochondria, the organelles that provide cells with energy, from passing them on to their children. Mitochondria contain their own DNA, and children inherit all of their mitochondria from their mother.

Researchers have studied the safety of one of three main types of mitochondrial replacement therapies, called ‘spindle transfer,’ which was used to make the first baby with genetic material from three people, who was born in Mexico in 2016. In this method, the nuclear DNA from the egg of a woman with faulty mitochondria is transferred to a donor egg with healthy mitochondria that has had its nuclear DNA removed. The egg is then fertilized with the father’s sperm in a test tube. The resulting embryo contains genes from both parents in addition to mitochondrial genes from the donor.

About one in 5,000 children are born with diseases caused by harmful mutations in their mitochondrial DNA. The faulty genes can lead to problems in many organs including the heart and the brain. So far, mitochondrial replacement therapy seems to be the most effective way of blocking these genes from being passed down to offspring.

By creating a safety study on spindle transfer, the results will provide important evidence to help regulators in countries where the process is not allowed assess its safety and efficacy, which in turn could advance the field.