Cysteine protease inhibitors are molecules that specifically block the activity of cysteine proteases, a type of enzyme that breaks down proteins. These inhibitors play a crucial role in regulating various biological processes, including cell signaling, immune responses, and protein turnover. They are found in many organisms, including plants, animals, and microorganisms. These inhibitors are important in medical research and drug development, as they can help control diseases where cysteine proteases are involved, such as certain cancers and infections. Examples of cysteine protease inhibitors include E64, a synthetic compound, and cystatins, which are naturally occurring proteins in the body.