Activation of caspases plays a central role in apoptosis. The CaspGLOWTM Red Active Caspase Staining Kit provides a convenient means for detecting activated caspases in living cells. The assay utilizes a caspase family inhibitor VAD-FMK conjugated to sulfo-rhodamine (Red-VAD-FMK) as the fluorescent marker. Red-VAD-FMK is cell permeable, nontoxic, and irreversibly binds to activated caspases in apoptotic cells.