The reduction of Metnase restores mitotic arrest in THP-1 cells. (A) CD34⁺ progenitors and THP-1 cells stably transduced with a vector control or the Metnase shRNA vector were treated with 10 or 25 μM ICRF-193 for 24 hours and then evaluated for the increase in G₂/M cell-cycle fraction by propidium iodide DNA staining. (B) Mitotic arrest was induced by 10 or 25 μM ICRF-193 for 4 or 18 hours in CD34⁺ cells, and THP-1 cells stably transduced with a vector control or the Metnase shRNA vector. Metaphase cells were imaged by tubulin immunofluorescence and DAPI nuclear morphology, and quantified as a percentage of the total cell population. (C) Mitotic arrest induced by 10 or 25 μM ICRF-193 for 4 or 24 hours in CD34⁺ cells, and THP-1 cells stably transduced with a vector control or the Metnase shRNA vector. Cells in mitosis were imaged by MPM-2 immunofluorescence and quantified as a percentage of the total population. Repressing Metnase increased the mitotic arrest of the THP-1 cells after ICRF-193 exposure so that they approximated the arrest of CD34⁺ cells. All experiments represent the average of at least 3 independent experiments, ± SEM. *Student t test (P < .05); **Student t test (P < .01).