Many genetic disorders are caused by faulty versions of a single gene. In the last decade, scientists have made tremendous strides in correcting these faults through “gene therapy”—using viruses to sneak in working versions of the affected genes.

But some disorders pose greater challenges. Down’s syndrome, for example, happens when people are born with three copies of the 21st chromosome, rather than the usual two. This condition, called trisomy, leads to hundreds of abnormally active genes rather than just one. You cannot address it by correcting a single gene. You’d need a way of shutting down an entire chromosome.

But half of us do that already. Women are masters of chromosomal silencing.

Women are born with two copies of