The US National Institutes of Health (NIH) has reaffirmed its ban on research that involves gene-editing of human embryos. In a statement released on April 29, NIH director Francis Collins spelled out the agency’s long-standing policy against funding such research and the ethical and legal reasons for it.

The statement comes after Nature’s report last week that researchers in China had used a gene-editing technology called CRISPR to remove disease genes from a human embryo. That research was published in Protein & Cell on April 18.

The NIH is concerned about the safety of the technique and the ethical implications of altering genes that will be passed to future generations of humans. Collins also pointed out that there are few clinical situations in which editing would be the only way to prevent the passage of a genetic disease from parent to child. In all but very rare cases, parents with a genetic disease could create embryos in vitro and screen them for the presence of the faulty gene.

Additionally, Collins wrote, a US law specifically bans the government from funding work that destroys human embryos or creates them for the purposes of research. The 1996 provision, known as the Dickey-Wicker amendment, was modified in 2011 to allow research in embryonic stem-cell lines.

The NIH says that the law's wording would probably prohibit funding for work in a nonviable human embryo, since it defines ”embryo” as anything derived from “fertilization, parthenogenesis, cloning, or any other means.” The research published in Protein & Cell used embryos that had been fertilized by two sperm, rendering them nonviable.

Unlike many other countries, the United States does not ban work in human embryos outright. While some US states do have such restrictions, others’ rules are less clear and some do not ban it at all, says bioethicist Hank Greely of Stanford University in California. In these states, researchers could carry on with private funding.

In his statement, Collins says that the question of editing embryos is not a new one, and is “viewed almost universally as a line that should not be crossed.” But not everyone agrees, especially when the work involves embryos that cannot develop into human beings.

“I am not in favor of the NIH policy and I believe that the Chinese paper shows a responsible way to move forward,” says David Baltimore, a biologist at the California Institute of Technology in Pasadena. “But it is the will of Congress that there be no work with human embryos and I assume that means even ones that are structurally defective.”

This article is reproduced with permission and was first published on April 29, 2015.