Take a cell from a woman and a cell from a man. Look at the X chromosome, which is big and packed with genes. She has two copies. He has one.
That looks like a problem, and it is one. Aleksey Shatskikh, a Candidate of Biological Sciences and senior researcher at the Laboratory of Functional Genomics of the Koltsov Institute of Developmental Biology, points out that the genes on the X have nothing to do with sex. They are the same in a man's body and a woman's. Yet women carry two sets of them, so on paper her cells could make twice as much of every protein those genes encode.
Why does that matter? Because a body expects each gene to work "no more than needed and no less." Double the dose, and the consequences are serious. For a woman, the stakes are her own beginning: if the fix breaks, an early embryo dies, which is what a miscarriage is. If it breaks later, it can show up as malformations, immune disease, and some cancers.
The fix is blunt. Her cells switch off one whole X chromosome.
Shatskikh's account of how is the part that sticks. A special non-coding RNA, a molecule that carries no recipe for a protein, picks which of the two X chromosomes will go quiet. Then it covers it. In his words, it wraps the chromosome in a kind of fur coat made of itself.
The RNA wraps the chromosome in a kind of fur coat and calls in repressive epigenetic machinery. That machinery adds repressive marks, and the whole chromosome is switched off.
Они ее оплетают такой шубой своеобразной из некодирующей РНК и привлекают к ней репрессивные эпигенетические механизмы, которые вносят репрессивные метки, и таким образом целая хромосома выключается.
— Aleksey Shatskikh, Scientific Russia, 14:57
The coat does not silence anything itself. It is a landing pad. The RNA arrives, spreads over the chromosome, and summons the cell's silencing crews, which stamp the chromosome with marks that say "do not read." One molecule, one choice, one chromosome shut down.
Researchers have since mapped parts of this machinery. The coat-making RNA is called XIST. A 2024 paper in Nature Structural & Molecular Biology, by Charbel Alfeghaly, reports that XIST sets off repressive histone modifications, chemical tags on the proteins that spool the DNA, and turns down most genes on the X, in a way that depends on a protein called SPEN.
A 2025 study in Nature Cell Biology, by Antonia Hauth, adds a twist about time. The silencing of so-called escapees, genes that slip past the shutdown, starts out fully reversible. Keep XIST turned up for long enough, though, and it becomes permanent, with the DNA of those escapees stamped with methyl groups.
Back to Shatskikh's point, and what it says about the cell. There is no committee deciding which chromosome to mute. An RNA floats about, finds its target by chemistry, binds it, and brings the right proteins along. Genes for non-coding RNAs are regulated much like ordinary genes, so even this coat is switched on by signals the cell receives.
So half of your X chromosomes, if you are a woman, are wearing a coat in every cell. It is the price of a fair dose.
