Cysteine peptidases 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 microorganisms, and are involved in numerous biological processes such as protein degradation, cell signaling, and immune responses. These peptidases are classified into different families based on their structure and function. Some well-known examples include papain, derived from papaya, and cathepsins, which are important in human health and disease. Understanding cysteine peptidases is essential for developing therapeutic strategies against various conditions, including cancer and inflammatory diseases.