Non-gel-based approach for analysis of expression proteomes. The most commonly used, commercially available, quantitation methods coupled with the MDLC approaches of comparative proteomics are depicted. The figure shows the ICAT and iTRAQ reagent structures. The peptide-reactive group (PRG) covalently links iTRAQ as well as the ICAT reagent isobaric tag with the lysine side chain and N-terminal group of peptide. The fragmentation shown with dashed lines occurs during MS/MS. The balance group of the iTRAQ reagent undergoes neutral loss, and the resultant reporter group (114–117) peaks in the low-mass region are used for relative quantitation. Reprinted from Tannu and Hemby  with permission of Elsevier.