(CIT): Neuropeptides perform a large variety of functions as intercellular signaling molecules. To study peptides using "proteomic" approaches, it is necessary to make several modifications in order to detect, identify, and quantify the peptides. For example, peptidomics studies usually analyze the native peptide forms in a tissue, not tryptic fragments, and this complicates sequence analysis. Also, postmortem degradation of proteins in a sample needs to be greatly reduced in order to allow detection of neuropeptides, which are typically present at very low levels. Quantitative peptidomic studies generally use differential isotopic tags to label two sets of extracted peptides, as done with proteomic studies, except that the thiol-based reagents typically used for quantitation of proteins are not suitable because most peptides lack Cys residues.