Canine Cystatin C (CST3) is a secreted cysteine protease inhibitor belonging to the type II cystatin family, which also includes Cystatin S, Cystatin SA, and Cystatin SN. In dogs (Canis lupus familiaris), cystatin C is produced by most nucleated cells and released into extracellular fluids such as blood and cerebrospinal fluid, where it functions to inhibit cysteine proteases including cathepsins B, H, K, L, and S. Through this activity, canine cystatin C helps regulate extracellular proteolysis, inflammation, immune responses, and tissue remodeling, protecting tissues from excessive protease-mediated damage. In canine health, cystatin C has been investigated as a biomarker of kidney function, since it is freely filtered by the glomerulus and metabolized by renal tubular cells, making circulating concentrations reflective of glomerular filtration rate (GFR). Dogs are also used as comparative animal models for human renal and cardiovascular diseases, and cystatin C has been studied in canine models of chronic kidney disease (CKD) to evaluate early detection of reduced kidney function and progression of renal pathology, providing insights relevant to both veterinary medicine and human nephrology.