The proprotein convertases are a group of enzymes, found in humans and other organisms, whose primary known function is to activate other proteins. In humans, there are presently nine recognized proprotein convertases. Each enzyme of this group operates by proteolytic cleavage of another protein's inactive form (the proprotein), after which the protein's activity is changed (usually increased). In some cases, the portion which is removed is also known to have biologic function. The proprotein convertases are generally felt to have overlapping, related but distinct functions from each other.