The document discusses collagenases and their role in plaque vulnerability and stability. It presents evidence that: 1) Macrophages in human and experimental atherosclerotic plaques express collagenases like MMP-1, MMP-8, and MMP-13 which can degrade collagen. 2) Lipid lowering in rabbits reduces macrophage accumulation and collagenase expression in plaques, while increasing collagen content, suggesting collagenases regulate collagen balance. 3) Mice with genetically reduced collagenase activity have altered collagen content in plaques, providing direct evidence collagenases regulate plaque collagen levels.