Trypsin is a digestive serine protease, made as a zymogen (proenzyme) in the pancreas, and activated by enterokinase as it enters the duodenum. It cleaves protein chains on the carboxy-terminal to arginine and lysine, unless proline is the amino acid residue on the other side of the cleavage site. Because it reliably cleaves both long and short peptide chains with well-defined specificity, trypsin is extensively used in protein sequencing protocols and other proteomic applications. It is also the usual enzyme of choice for passaging cells in culture. Most cells tolerate a short exposure to trypsin solutions; the proteolysis is normally terminated as soon as cells have separated from their solid substrate and from each other. After trypsinization, cells experience a boost in mitosis, especially if they are replated somewhat sparsely. Other biological procedures that require removal of extraneous protein utilize trypsin, including isolation of mitochondria from some tissues, and some immunohistology protocols.