Mass spectrometry is the major analytical tool for the identification and quantification of proteins
in biological samples. In so-called top-down proteomics, separation and mass spectrometric
analysis is performed at the level of intact proteins, without preparatory digestion steps. It
has been shown that the tandem mass tag (TMT) labeling technology, which is often used
for quantification based on digested proteins (bottom-up studies), can be applied in top-down
proteomics as well.