A Wang

In an orchestra that has been playing the same symphony for years, the musicians gradually start to drift: each one plays on their own, yet the notes remain the same. That is roughly what happens to cells as they age. Scientists have come up with a way to measure this discord, that is, how faithfully each cell follows the shared program of gene expression.

A mouse gets food that is missing a single amino acid, and within five days it loses a fifth of its weight. Its white fat turns darker, as if tiny stoves had lit up inside it. This is not hunger and not exercise: cysteine was simply removed from the diet. Cysteine is a building block of proteins that our body can make on its own from another amino acid, methionine.