What is a chemical probe? A chemical probe is simply a reagent—a selective small-molecule modulator of a protein’s function—that allows the user to ask mechanistic and phenotypic questions about its molecular target in cell-based or animal studies. Importantly, a chemical probe is not a key that interacts with only one lock (the protein target) in a cell no matter what concentration it is applied. It is a molecule that is governed by the same rules of chemistry as any other; therefore, experiments using chemical probes must be designed and interpreted with care.