['Cell division is an inevitable and vitally indispensable event in cell life, when the nucleus and cytoskeleton undergo profound reorganization. Cytoplasmic division (cytokinesis) is known to occur immediately after the end of nuclear division, when the nuclear envelope breaks down, and chromosomes condense and segregate, but its driving mechanism remains enigmatic. Myosin, particularly myosin-II, is thought to be required for cytokinesis as a force-generating element, the activity of which is mainly regulated through phosphorylations on its 20-kDa regulatory light chains (RLCs).', 'Multiparameter flow cytometric analysis was performed on fixed HeLa S3 cells (suspension culture cells) sequentially stained with the polyclonal antibody (termed PP1) against both phosphorylated sites (serine-19 and threonine-18) on the RLC, and with propidium iodide for DNA. "Positive" cells were sorted, followed by their microscopic examination. Fluorescence microscopy was employed to visualize the cell-cycle-dependent distribution of immunolabeled diphosphorylated RLCs in both HeLa S3 and adherent HeLa cells.'] Is phosphorylation at threonine-18 in addition to phosphorylation at serine-19 on myosin-II regulatory light chain a mitosis-specific event?