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 the catalytic process. These enzymes are found in various organisms, including humans, and are involved in important biological functions 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 and as targets for drug development.