Cysteine-mediated protein activities comprise proteases, kianses, oxidoreductases and metabolic enzymes that rely on cysteine residues for catalysis and regulation. These functional cysteine residues demonstrate heightened reactivity relative to non-functional cysteines, and are sensitive to a myriad of oxidative protein modifications that serve to regulate protein activity in vivo. We are applying chemical proteomic technologies to identify cysteines that are susceptible to posttranslational modifications such as S-nitrosylation, with the goal of discovering and functionally characterizing novel sites of protein regulation. We are also developing libraries of cysteine-reactive small molecules to selectively perturb subsets of cysteine-mediated protein activities in cells. We hope these small molecule probes and mass spectrometry techniques will allow us to identify and modulate disease-relevant proteins that rely on key cysteine residues for function.