Scientists should refrain from studies that alter the genome of human embryos, sperm, or egg cells, researchers warn in a commentary published today in Nature.

In it, they sound the alarm about new genome-editing techniques known as CRISPR and zinc-finger nucleases that make it much easier for scientists to delete, add, or change specific genes. These tools have made it possible to make better animal models of disease and more easily study the role of individual genes. They also hold the promise of correcting gene mutations in patients, whether in blood cells, muscle cells, or tumor cells.

But scientists have also used the technology to make genetically altered monkeys. And there are rumors that some researchers are trying the same technique on human embryos, MIT Technology Review reports.

That is unsafe and unethical, say Edward Lanphier and four other researchers in their commentary. Ethically justifiable applications “are moot until it becomes possible to demonstrate safe outcomes and obtain reproducible data over multiple generations,” they write. They call for a moratorium on any experiments that would edit genes in sperm cells, egg cells, or embryos while scientists publicly debate the scientific and ethical consequences of such experiments. The recent discussion of mitochondrial DNA replacement therapy in the United Kingdom could be a model, they suggest.

They hope that such a discussion would help the public understand the difference between genome editing in a person’s somatic cells—cells other than sperm and egg cells—and editing in cells that could pass the changes on to future generations, says Lanphier, who is president and CEO of Sangamo BioSciences in Richmond, California, a company that hopes to use gene-editing technology to treat patients. “There’s an important and clear ethical boundary between genome editing in somatic cells versus in the germ line.”

George Daley, a stem cell researcher at Boston Children’s Hospital and Harvard Medical School, agrees that a public debate is important. Among scientists, he says, there is broad consensus that at the moment “it’s far too premature and we know far too little about the safety to make any attempts” at modifying germ cells or embryos. But that will eventually change, he says. “There needs to be broad public debate and discussion about what, if any, are the permissible uses of the technology.”