Â{u0060}key experiment in biomedical research is monitoring the expression of different proteins in order to detect changes that occur in biological systems under different experimental conditions. Â{u4A25} method that is most widely used isÂ{u4A25} Western blot analysis.Âרile Western blot is a workhorse in laboratories studying protein expression and has several advantages, it also has a number of significant limitations.Â{u026E} particular, the method is semi-quantitative with limited dynamic range.Âץstern blot focuses on a single protein per sample with only a small number of representative samples analyzed in an experiment.ÂYw quantitative tools have been needed for some time to at least supplement, & possibly replace, the Western blot. Mass spectrometric methods have begun to compete with Western blot for routine quantitative analyses of proteins.Â{u03EE}e of these methods is based on the tandem mass spectrometry technique of selected reaction monitoring (SRM), which is also called multiple reaction monitoring (MRM). Â{u396C}ected reaction monitoring is actually an older tandem mass spectrometry technique, first described in the late 70s, that is widely utilized in the quantitative analysis of small molecules like drugs & metabolites.The use of selected reaction monitoring for the quantitative analysis of proteins has a number of advantages.Â{u036F}st importantly, it is fundamentally quantitative with a wide dynamic range.The output of the analysis is a numerical result that can range over several orders of magnitude.Â{u03F4}her advantages include sufficient specificity & sensitivity to detect low abundance proteins in complex mixtures.Â{u01A9}nally, selected reaction monitoring can be multiplexed to allow the quantitative analysis of relatively large numbers of proteins in a single sample in a single experiment.Â�This Brief will explain both the theoretical & experimental details of the selected reaction monitoring experiment as it is applied to proteins.