Proteins found in nature vary greatly in size from 5 kDa to greater than 400 kDa. While it is possible to study intact proteins by mass spectrometry (MS) and the modifications present on these proteins, the most common proteomic approaches currently utilize digestion with site-specific proteases to generate smaller fragments, peptides, as a first digestion with site-specific proteases to generate smaller fragments, peptides, as a first step in the analyses (protein digestion). Peptides are easier to characterize and can be separated using reverse phase supports by high performance liquid chromatography (HPLC) using a C18 column. HPLC-coupled to a Tandem MS is used to obtain fragmentation data of individual peptides. This digestion of proteins into smaller pieces is typically carried out by proteases such as trypsin (NEB #P8101) and Endoproteinases GluC (NEB #P8100) and AspN (NEB #P8104).