Human females have two X chromosomes, and males only one. This chromosome imbalance also extends to other branches of the animal kingdom. Interestingly, the humble fruit fly has devised a different way to 'equalize' these differences. Whereas human and mouse females shut down one of their X chromosomes, in fruit flies it instead the male doing the work. An epigenetic factor known as the MSL complex binds to the single male X chromosome and uses its histone acetylation function to hyperactivate the X to try to reach RNA production levels equivalent to those achieved by the two X chromosomes carried by females. If this process fails, male flies die.