Cystatin S is a type of protein known as a cysteine protease inhibitor. It plays a crucial role in regulating enzymes that break down proteins, helping to maintain balance in various biological processes. This protein is primarily found in human saliva and is produced by salivary glands. Research has shown that Cystatin S may have protective effects against certain diseases, including Alzheimer's disease and cancer. Its ability to inhibit specific enzymes can help prevent tissue damage and inflammation, making it an important focus in medical studies aimed at understanding and treating these conditions.