Z-Phe-Lys-AOMK is a synthetic peptide-based inhibitor primarily used in biochemical and medical research to selectively target and inhibit cysteine proteases, especially cathepsins such as cathepsin B, K, and S. These enzymes are involved in protein degradation and are overexpressed in certain pathological conditions including cancer, osteoporosis, and inflammatory diseases. One of the key applications of Z-Phe-Lys-AOMK is in the study of protease activity in cellular systems. It acts as an irreversible inhibitor by forming a covalent bond with the active site thiol group of the target enzyme, allowing researchers to probe enzyme function and validate protease roles in biological processes. It is also used in activity-based protein profiling (ABPP), a technique that enables the monitoring of active enzymes in complex biological samples. Due to its specificity and cell permeability, Z-Phe-Lys-AOMK helps label and detect active cathepsins in live cells or tissue extracts, supporting drug discovery and biomarker development. Additionally, this compound serves as a useful tool in developing and testing therapeutic inhibitors, particularly for diseases where cathepsin overactivity contributes to tissue degradation or tumor invasion.