This approach can be used to quantify relative protein expression levels based on the relative intensity of the heavy and light forms of ICAT-labeled cysteines containing tryptic peptides as measured by tandem MS. Cysteine-specific modification and affinity purification achieves a reduction in proteome complexity. The ICAT technique has been widely applied to quantitative proteome analysis of a variety of sample types, such as cultured cells, tissues, and body fluids. Subsequently, a second generation of ICAT reagent was developed that contains an acid-cleavable moiety for removing the affinity tag. There are numerous articles and reviews [2-8] on the methodology and applications of ICAT technology, summarized here.