We are a small step closer to the day when two women or two men could have biological children of their own , thanks to improved methods for creating mice with same-sex parents. But the work also shows that there is an enormous amount still to do before this could be attempted in humans.

“It is never too much to emphasise the risks, and the importance of safety, before any human experiment is involved,” says Wei Li at the Chinese Academy of Sciences in Beijing. “But we think our …