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For a person with two X chromosomes, full expression of the genes on both could mean a disastrous double dose of their protein products, interfering with the delicate balance of protein expression and interactions all over the body. But cells have a simple solution: turn off one X chromosome and crumple the extra genes into a quieted mass of DNA called a Barr body. A long noncoding RNA known as XIST (pronounced “exist”), which is expressed from the “inactive” X itself, plays a key role in this process. Acting only on the chromosome it’s transcribed from, XIST coats the DNA, turning it into silent heterochromatin.
Except there’s a problem. Certain genes just won’t stay mum.
Some Barr body genes, including XIST itself, actively control the silencing of other stretches of the X.
Scientists had long known that some genes on X chromosomes aren’t subject ...
