Sometimes the future arrives in waves, advancing abruptly and then withdrawing. Last week, a Chinese researcher named He Jiankui announced that he had successfully altered the genetic code of a pair of twin girls born last month. He said that while they were still embryos, he had edited the babies' genes to make them resistant to HIV infection, but he offered few further details.

Scientists and bioethicists worldwide were incensed by Dr He's announcement, given serious concerns about the danger the still-developing technology could pose to humans. Jennifer Doudna, a researcher at the University of California at Berkeley who helped develop the gene-editing technique known as Crispr, said she was "deeply disappointed" and "a bit horrified" by what Dr He had done, adding that his intervention was not medically necessary and breached international guidelines on the use of gene-editing technology.