A proenzyme is the inactive precursor form of an enzyme, also known as a zymogen, which the body or a microbe produces and later switches on when and where its activity is needed. Keeping the enzyme dormant during synthesis and transport prevents it from digesting the tissues that made it. Activation usually involves cutting away part of the protein chain or a shift in conditions such as pH. Classic examples are the digestive proenzymes pepsinogen, secreted by the stomach and activated to pepsin by acid, and trypsinogen, released by the pancreas and converted to trypsin in the small intestine. Food processing borrows the same idea: rennet, for instance, contains the milk-clotting enzyme chymosin generated from its precursor prochymosin in the calf stomach.