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 function. These enzymes are found in various organisms, including humans, plants, and bacteria, and are involved in many biological processes such as digestion, immune response, and cell signaling. These proteases are classified into different families based on their structure and function. Some well-known cysteine proteases include papain, found in papaya, and cathepsins, which are important for protein degradation in lysosomes. Their activity is tightly regulated, as imbalances can lead to diseases, including cancer and neurodegenerative disorders.