Leading scientists have called for a global moratorium on the use of powerful DNA editing tools to make genetically modified children.

The appeal was made in the journal Nature by a group of 18 scientists from seven countries (Canada, China, France, Germany, Italy, New Zealand and the U.S.), including some leaders in the development of CRISPR technologies, the most precise tool available for modifying DNA. The signatories have called for a voluntary moratorium on all studies involving gene editing of human eggs, sperm or embryos — so-called germline cells. Scientists proposed a similar moratorium in 2015, but in this newer version, the scientists go further, asking not just individual researchers to agree to stop work on gene editing human germline cells, but calling on nations to create explicit laws or regulations to prevent such studies for now, and to develop a framework for allowing the studies when they deem they are safe and acceptable.

The call for a moratorium received strong backing from the US National Institute of Health. “We have to make the clearest possible statement that this is a path we are not ready to go down, not now, and potentially not ever,” the NIH director, Francis Collins, told the Guardian.

The call comes four months after a team of scientists in China used CRISPR to modify the DNA of healthy twin human embryos in a bid to genetically “vaccinate” them against HIV infection.