If defective genes make someone sick, why not just edit out the malfunctioning versions and add in ones that work? That’s the idea behind gene therapy — but it hasn’t lived up to its promise.
For one thing, controlling where a gene is added into the genome is difficult, and randomly inserted genes can disrupt others, fouling up their functions or causing cancers. Also, current gene-editing methods don’t completely shut down bad genes.
A technique based on the immune systems of bacteria and archaea, another kind of microorganism, may soon provide solutions.