Cystatin C, a 13-kDa cysteine proteinase inhibitor, is secreted by all cell types and excreted via glomerular filtration, serving as an early, sensitive renal dysfunction biomarker. A Cystatin C mutation links to amyloid angiopathy. Its expression in vascular smooth muscle cells is reduced in both atherosclerotic and aneurysmal aortic lesions, indicating a role in vascular disease.