The recent introduction of isobaric peptide tags for relative and absolute quantification (iTRAQ) of proteins in different samples was a major breakthrough in quantitative proteomics. The iTRAQ method is based on the differential covalent labelling of peptides from proteolytic digests with one of four iTRAQ reagents resulting in the incorporation of 144.1 Da to peptide N-termini and lysine residues. Peptides with different tags are indistinguishable by mass but can be differentiated by collision-induced dissociation (CID) (normally applied during MS peptide sequencing) through release of a reporter ion, each of which has a different mass (114.1, 115.1, 116.1 or 117.1 Da). The analysis of the intensity of reporter ions allows the simultaneous sequencing and quantification of labelled peptides.