The Great Escape

摘要

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.