An early-stage human embryo, as seen through a microscope in an IVF lab.

Since its invention four years ago, a powerful and precise technology for editing DNA called CRISPR has transformed science because of how it makes altering the genetic makeup of plants and animals easier than ever before.

But no possibility opened by gene-editing technology has been so exciting, frightening, or as hotly contested as its capacity to allow humanity, for the first time, to control the genetic constitution of children by applying CRISPR to human embryos, sperm, or eggs—cells which together make up the “germ line.”

On Tuesday, in a striking acknowledgement that humanity is on the cusp of genetically modified children, a panel of the National Academy of Sciences, the nation’s source of blue-ribbon advice on science policy, recommended that germ-line modification of human beings be permitted in the future in certain narrow circumstances to prevent the birth of children with serious diseases.

"Heritable germline genome editing trials must be approached with caution, but caution does not mean that they must be prohibited," according to a 216-page report released today and which was researched and written over the course of a year by a 22-member panel of prominent scientists and experts.

The recommendations came freighted with moral and technical caveats, however. The panel believes it will be many years before germ-line engineering is safe enough to consider. The panel also said it should proceed only under “stringent oversight,” and drew a bright line between preventing disease and “enhancements” like attempting to alter genes to make people more intelligent, which it said should not be pursued “at this time.”

Despite the cautious language, the panel’s endorsement of GM humans could prove politically explosive, and puts the academy’s experts in conflict with existing legislation in Europe and the U.S. as well as with swaths of the public who oppose the idea of modifying the human genome from birth out of religious conviction or for other reasons.

Germ-line modification is already prohibited as a practical matter in the U.S. In 2015, pro-life legislators added a rider to the U.S. Department of Health and Human Services appropriations bill, which forbids the U.S. Food and Drug Administration from considering any proposal to create genetically modified offspring.

The legislation, which has to be renewed periodically, means that any proposal to modify an embryo and create a child would be ignored and could not legally proceed in the U.S.

In contrast, the academy panel argued that germ-line editing should be allowed in narrow cases where it is the only option for “preventing a serious disease or condition.” For instance, a couple who each suffers from beta thalassemia might only have healthy children free from the inherited blood disorder if they were able to produce embryos in which the genetic defect was corrected using gene editing. The report acknowledges that such circumstances might be exceedingly rare.

“They show a narrow but clear path to future clinical use,” says Tetsuya Ishii, a bioethicist at Hokkaido University in Japan who tracks global legislation on germ-line modification. He says the report also provides a justification for laboratory research already occurring in China, Sweden, and the U.K. in which gene-editing is being applied to human embryos to explore its potential. “They want to show that basic research toward severe disease prevention would be permissible,” he says.