Fan's team writes in the paper that proof-of-principle experiments for human-embryo editing such as theirs are important to conduct while the ethical and legal issues of germline modification are being hashed out. “We believe that any attempt to generate genetically modified humans through the modification of early embryos needs to be strictly prohibited until we can resolve both ethical and scientific issues,” they write.

Daley sees a stark contrast between Fan’s work and research approved in February by UK fertility regulators that will allow CRISPR genome editing of human embryos. Those experiments, led by developmental biologist Kathy Niakan at the Francis Crick Institute in London, will inactivate genes involved in very early embryo development, in hopes of understanding why some pregnancies terminate. (The work will be done in viable embryos, but the researchers' licence requires that experiments be stopped within 14 days.)

Earlier this year, developmental biologist Robin Lovell-Badge, also at the Francis Crick Institute, toldNature that he thought that the carefully considered UK approval might embolden other researchers who are interested in pursuing embryo-editing research. “If they've been doing it in China, we may see several manuscripts begin to appear,” he said.

Whereas Niakan's work is answering questions intrinsic to embryology, Fan's work is establishing proof of principle for what would need to be done to generate an individual with resistance to HIV, Daley adds. “That means the science is going forward before there’s been the general consensus after deliberation that such an approach is medically warranted," he says.