Tachpyridine induces cell-cycle arrest in the G₂ phase of the cell cycle. HeLa cells treated with 7.5 μM tachpyridine for 18 hours were double-labeled for the M-phase marker phosphorylated H3 and for DNA content, followed by flow cytometry analysis. Untreated controls (top) show 12% of cells in G₂/M by propidium iodide (PI) staining and 2.2% of those cells in mitosis. Cells treated with 50 ng/mL nocodazole, an M-phase arrestor, show 96% of cells in G₂/M by PI staining and 87% of those cells in mitosis (middle). Tachpyridine-treated cells (bottom) display 41% of cells in G₂/M by PI staining, and only 0.03% of cells in mitosis, consistent with the induction of a G₂- and not M-phase arrest.