Really like this analogy: It is a somewhat astonishing fact of life that the exact same DNA is shared by every cell in your body, from the skin to the brain; those cells differ in appearance and function because, in each of them, a molecular gizmo “transcribes” some DNA segments rather than others into molecules of single-stranded RNA. These bits of RNA are in turn used as the blueprints for proteins, the molecular machines that do most of a cell’s work. If DNA is your phone’s home screen, then transcription is like tapping an icon. By sampling the RNA present in a group of cells, researchers can see which programs those cells are running at that moment; by sampling it after the cells have been infected with a virus, they can see how that virus substitutes its own software.