– (S)-nitro-Blebbistatin is a more stable form of (-)-blebbistatin, which is a selective cell-permeable inhibitor of non-muscle myosin II ATPases. (-)-Blebbistatin rapidly and reversibly inhibits Mg-ATPase activity and in vitro motility of non-muscle myosin IIA and IIB for several species (IC50s = 0.5-5 μM), while poorly inhibiting smooth muscle myosin (IC50 = 80 μM). Through these effects, it blocks apoptosis-related bleb formation, directed cell migration, and cytokinesis in vertebrate cells. However, prolonged exposure to blue light (450-490 nm) results in degradation of blebbistatin to an inactive product via cytotoxic intermediates, which may be problematic for its use in fluorescent live cell imaging applications. The addition of a nitro group stabilizes the molecule to circumvent its degradation by prolonged blue light exposure. (S)-nitro-Blebbistatin has the same stereochemistry as the active (-)-blebbistatin enantiomer.