The Mass Spectrometry Core identifies and quantifies proteins that change expression in well-characterized protein fractions from cancerous cells or tissues. This includes identifying and quantifying changes in binding partners and post-translational modifications. Column chromatography and gel electrophoresis-based one and two-dimensional separations of protein complexes coupled to mass spectrometry are used. Techniques such as difference gel electrophoresis (DIGE), isobaric tag for relative and absolute quantitation (iTRAQ) and 18O-labeling as well as non-labeling methods (MudPit, multi-dimensional protein identification technology) are available for quantifying relative differences in protein expression and post-translational modifications. We developed methods to detect post-translational modifications such as LCMS methods to accurately determine the intact mass of proteins, selective fluorescent labeling of S-nitrosothiols (S-FLOS) to detect nitrosated cysteines in proteins, and ion mapping methods to map post-translational modifications that produce a signature mass or mass difference when the modified peptide is fragmented.