, or , are
enzymes secreted in inactive form.
Under certain conditions a zymogen
shifts to the active form of the enzyme.
Zymogen secretions in general happen
because the enzyme activity can harm
the secretory tissue.

For example, the pepsinogen
by the stomach becomes active under
pH turning into the enzyme pepsin.
Other well-known zymogens are
trypsinogen and chymotrypsinogen,
enzymes that are secreted by the
exocrine and which become
and chymo respectively.