A zymogen is an inactive enzyme precursor. A zymogen requires a biochemical change, such as a hydrolysis reaction revealing the active site, for it to become an active enzyme.

Examples of zymogens are the pancreatic trypsinogen which becomes trypsin and chymotrypsinogen which becomes chymotrypsin. These zymogens originate from the endoplasmic reticulum and are moved via vesicular sorting to a lysosome or vacuole. Hydrogen ions are then pumped into the organelle to trigger the zymogens to their functional forms.

The proteasome is also a zymogen. The catalytic beta subunits are cleaved to reveal the reactive threonine.

In the stomach, parietal cells release pepsinogen, also a zymogen activated by the hormone gastrin. Pepsin is stored as pepsinogen so it will only be released when needed, and does not digest the proteins in the stomach's lining.