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