iTRAQ Reagents incorporate a "tagging" strategy that facilitates discovering and quantifying protein expression changes by mass spectrometry (MS) across multiple samples in studies ranging from biomarker discovery to disease analysis. The reagents address 3 key challenges: increased multiplexing of samples, broad proteome coverage, and simultaneous identification and quantitation from a mass spec experiment. Using the technology, researchers can improve overall protein and proteome coverage, simultaneously compare multiple samples, (i.e. normal versus diseased verses drug-treated or time course studies) in the same experiment; quantify, in absolute terms, specific proteins of interest, such as biomarkers, or screen drug targets, and increase statistical relevance. The reagents are isobaric, which confers the ability to run multiples of 4 or more samples in one experiment and where all 4 samples behave as one in separation chromatography and mass spec. This enables researchers to pull out low abundant marker proteins, because all signal in MS using iTRAQ is additive.