Humans have two copies of each chromosome, but Down syndrome occurs when there are three copies of chromosome 21. In this situation, the extra genes cause developmental problems. However, every female in the world is already equipped with a genetic tool to naturally switch off unnecessary chromosomes.
Since females have two X chromosomes (XX), they have twice as many X-linked genes as males. Females balance this out by using an Xist gene (located on the X chromosomes) to stimulate the production of ribonucleic acid (RNA), which coats almost the whole chromosome, effectively deactivating it.
Researchers copied this genetic utility by inserting a copy of the Xist gene into the extra chromosome in cells from people with Down syndrome. The gene silenced the chromosome, and stayed that way for several generations of cells. While genetic therapy that could remedy chromosome 21 in every cell in a whole body is still a very long way off, this discovery is an enormous step forwards.