The cell cycle is the process in which a growing cell duplicates all of its DNA, copying each base precisely once, and then divides into two daughter cells. It consists of four phases: S, for synthesis, when the new DNA is made; M, for mitosis, when the cell splits into two; and two resting gap phases separating them, G1 before S phase and G2 between S and M.

The cell cycle is tightly regulated; ensuring that cells proceed through it at the proper pace and at the proper time is essential to maintaining an organism’s healthy development. But the cycle can be broken. Cancer is the ultimate example of the cell cycle gone awry, as cancer cells divide uncontrollably, often picking up DNA damage as they go.

In a laboratory setting, it’s often useful to know which phase of the cell cycle one’s specimens are currently in. A researcher might want to know if all the cells in a dish are growing synchronously, for example, or to confirm that the cells she treated with her favorite drug are in fact stopping the cycle as expected.