Activation of caspases plays a central role in apoptosis. The CaspGLOW™ Fluorescein Active Caspase-9 Staining Kit provides a convenient means for sensitive detection of activated caspase-9 in living cells. The assay utilizes the caspase-9 inhibitor LEHD-FMK conjugated to FITC (FITC-LEHD-FMK) as a fluorescent marker. FITC-LEHD-FMK is cell permeable, nontoxic, and irreversibly binds to activated caspase-9 in apoptotic cells.