Nobel laureate David Baltimore of CalTech speaks at the National Academy of Sciences international summit on the safety and ethics of human gene editing on Tuesday in Washington. (Susan Walsh/AP)

Leaders of an international conference warned Thursday that it would be “irresponsible” to use a new gene-editing technique on human embryo, sperm or egg cells until questions of safety are addressed and there is “broad societal consensus” on the purpose of the effort.

The criteria for clinical research or therapeutic use of that type of gene alteration haven’t been met yet, the organizing committee of the International Summit on Human Gene Editing wrote in a statement issued at the end of the three-day conference. “The safety issues have not yet been adequately explored” and the prospects for even the most beneficial uses are currently limited, they said.

The conference at the National Academies of Sciences was called because a revolutionary way of editing genes — known as CRISPR-Cas9 — allows many scientists to alter genes in plants and animals quickly and inexpensively.

The technology, which takes advantage of the way bacteria defend against viruses, was developed in just the past four years.

The meeting was led by U.S., Chinese and British science and medical academies. Chinese scientists made news earlier this year when it was revealed that they had been applying gene-editing to nonviable human embryos.

The summit statement acknowledged the many “promising and valuable” uses for gene editing of somatic cells — which don’t transmit the human genome to succeeding generations. In a single individual, for example, proposals to alter genes to eliminate sickle cell anemia or strengthen immune cells’ ability to fight cancer can be undertaken within existing rules, the statement said, though scientists need to understand the risks and benefits of each therapy.

But modifying genes in “germline” cells involved in human reproduction would pass the alterations to all the cells of resulting offspring and into the human gene pool. That, the summit suggested, is a step that should not yet be taken, even to eradicate heritable diseases.

They said that risks include inaccurate gene editing that might produce unwanted mutations; the difficulty of predicting harmful effects; the difficulty of removing any harmful modifications from the gene pool; inequality that might result from enhancing one group of people but not another; and “the moral and ethical considerations in purposely altering human evolution.”

If genes in embryos or germline cells are edited for research only, the organizers added, “the modified cells should not be used to establish a pregnancy.”

As scientific knowledge advances and society’s views evolve, the statement added, these questions should be revisited. An ongoing forum for discussion of possible uses should be established by the international hosts, with representation for scientists, ethicists, patients, health-care providers, funders, faith leaders, private industry and others.