H Wang

In the bone marrow, as in a barracks, there is a constant roll call. Rookie T cells wait for their first assignment, and for a while they all look alike. But with age, soldiers with a special mark on their uniform start to appear among them: the protein annexin A1. This mark is not a decoration but a brake shoe that keeps the cell from charging into battle.

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.