Proteins that end in "-ogen" are zymogens, and proteins that start with "pro-" are what I call pro-proteins... (is there a more technical term?). Examples: pepsinogen/fibrinogen vs. prothrombin Anyway, what is the difference between how these two protein precursors work? From what I've been reading, it seems that they both are cleaved to release the active form of the protein. But there must be something different, right? Otherwise, why the different nomenclature, if not to confuse people with synonyms? EDIT: Isn't there also a distinction between "pre-" and "pro-"?