Cysteine proteases are a type of enzyme that break down proteins by cleaving peptide bonds. They contain a cysteine residue in their active site, which plays a crucial role in their catalytic activity. These enzymes are found in various organisms, including humans, plants, and bacteria, and are involved in numerous biological processes such as protein degradation, cell signaling, and immune responses. These proteases are classified into different families based on their structure and function. Some well-known examples include papain, found in papaya, and cathepsins, which are important for protein turnover in human cells. Cysteine proteases are also studied for their potential roles in diseases, making them targets for therapeutic interventions.