Caspase-9 complexed with an inhibitor, molecular model. Caspase-9 is a protease, an enzyme that cleaves proteins, that plays a role in apoptosis (programmed cell death). It also activates other caspases as part of the caspase cascade of apoptosis. Caspase-9 has been found to be critical for UV-induced apoptosis in skin cells and therefore, for the prevention of skin cancer.