One dad, two moms… on the cellular level. Now, legal. Almost.

Nature reports on a British legal ruling that’s a world-first, a step toward allowing medical scientists to create “three-parent” embryos:

This technique, known as mitochondrial replacement or three-person in vitro fertilization, aims to prevent women passing on harmful mutations in their mitochondria, the cell’s energy-producing structures. An estimated 1 in 5,000 children are born with diseases caused by such mutations, which typically affect power-hungry tissues such as the brain, heart and muscles. All mitochondrial DNA is inherited from the mother, and some women carry harmful mitochondrial mutations without experiencing symptoms themselves.

“It’s great news for the patients with mitochondrial disease. It gives them real hopes and that’s just fantastic,” says Doug Turnbull, a neurologist at Newcastle University, UK, who has led the effort to bring mitochondrial replacement to the clinic.

Proponents of the law change — a who’s who of UK science that included Nobel prizewinners, policy-makers and directors of leading science funders — had swarmed UK media with open letters and signed statements calling on Parliament to act. The opposition was more disparate — although no less vocal — and it included clergy representatives from the Church of England and the Catholic Church, secular advocacy groups, as well as some researchers.

Proponents of allowing mitochondrial replacement in other countries say that today’s vote could serve as a watershed leading to the technique’s eventual approval elsewhere. The US Food and Drug Administration currently bans such procedures, but it held a meeting last year to consider the scientific data needed to lift the ban. The Institute of Medicine, which advises the US government, plans to issue a report on the ethics of mitochondrial replacement.