Multiple reaction monitoring (MRM) is an MS/MS experiment that uses two sequential stages of independent mass analysis. In the product ion MS/MS scan, a precursor ion is selected by mass with the first mass analyzer, and the fragment ions formed as a result of collision-induced  dissociation are measured with a scan of the second mass analyzer and recorded in a mass spectrum. In an analogy to selected ion monitoring, if both mass analyzers in an MS/MS  instrument are set on a specific mass, the signal represents the precursor-to-product ion transition for a specific ion pair. This experiment is called reaction monitoring. If several  different precursor–product ion pairs are monitored, as is most often the case, the experiment is multiple reaction monitoring.