He Jiankui tried to use CRISPR to make individuals immune to HIV , but many believe the most-compelling case for gene editing before birth is to prevent genetic disorders.

Most disorders caused by mutations in a single gene, such as cystic fibrosis, can already be prevented. One way of doing this is to screen IVF embryos for harmful mutations before implanting them.

However, such preimplantation genetic diagnosis has limitations, particularly when you only need to inherit the mutation from one parent to …