In April 2015, in the first known use of Crispr on human embryos, researchers led by Junjiu Huang of Sun Yat-sen University in China tried to correct a defective gene that causes a blood disorder known as beta thalassemia. Though the experiment was ethically defensible — all the embryos were unviable because of a fatal defect — it also demonstrated the possible dangers of the technique because of the many things that went wrong. It was this experiment that gave urgency to the steps leading to the three academies’ call for a worldwide moratorium on modifying the human germ line.

David Baltimore, a leading biologist at the California Institute of Technology who helped organize the moratorium, said the proposed experiment appeared to be consistent with the principles laid out by the academies. Many such experiments are impossible for government-funded researchers in the United States because of the congressional ban, but “luckily, private and state funding sources are available to carry forward such work,” Dr. Baltimore said.

George Q. Daley, a stem cell biologist at Boston Children’s Hospital, said Dr. Niakan’s study of human embryos was “critical because we know them to be quite different from embryos of mice” and other mammals studied in laboratories. Congress’s restriction on human embryo research, he said, “puts us at a competitive disadvantage” with respect to Britain, “where many major discoveries have been made in human development.”