Researchers have gathered evidence that dangerous plaques in blood vessels can rupture by overproducing protein-digesting enzymes. Such ruptures can lead to artery-blocking clots. Almost everyone over age 60 has arterial plaque. Ruptures are what convert atherosclerosis from an indolent disease to a life-threatening emergency. The findings suggest ideas for diagnosing vulnerable plaques and developing new approaches to preventing and treating heart attacks and strokes by decreasing production of protein-destroying enzymes in vulnerable plaques or blocking their actions.