A chromatin twist to silencing choice

Although cells typically transcribe both copies of a particular gene, they sometimes flip off one copy and rely on the other. A histone-modifying protein might help determine which copies are turned on or off by tweaking the structure of chromatin, as Alexander et al. show.

This silencing—called monoallelic expression—reaches an extreme in female mammals, in which one X chromosome almost completely shuts down. Which copy a cell chooses to switch off appears to be random, and the selection mechanism remains unexplained.

Last year, the researchers showed that, in embryonic stem cells, would-be active and inactive X chromosomes differ even before one gets silenced. When the scientists tagged specific genes on the chromosomes using fluorescence in situ hybridization (FISH), one X chromosome typically carried two...