From the New Scientist: "when young mice are learning, a molecular fragment known as an acetyl group binds to a particular point on the histone protein that DNA wraps itself around - with the result that the cluster of learning and memory genes on the surrounding DNA ends up close to the acetyl group. ... This acetyl "cap" was missing in the older mice that had been set the same tasks. From this, the team concludes that the cap acts as an "on" switch for the cluster of learning and memory genes: removing the cap switches off the genes. Next, by injecting an enzyme known to encourage caps to bind to any kind of histone molecule, [researchers] artificially flipped the switch to the on position in old mice. The acetyl group returned to the histone molecule and the mice's learning and memory performance became similar to that of 3-month-old mice. ... it is still not clear why the switch flips off as we get older. One possibility is that it might help us cope with other cellular assaults that come with ageing, such as oxidative stress, [which] would mean that switching it on might have damaging side effects."