Proteomics is a large-scale comprehensive study of a specific proteome, including information on protein abundances, the variations and modifications, as well as their interacting partners and networks. In discovery proteomics, proteome analysis can be performed in two different strategies, bottom-up and top-down approaches, respectively. In the bottom-up approach, a crude protein mixture undergoes protease digestion first, and then separation by liquid chromatography, followed by MS analysis. In the top-down method, proteins are characterized by MS without prior proteolysis. This type of approach has the advantage of providing greater sequence coverage, resolution of sequence ambiguities and preservation of PTMs.