Per2 induces growth arrest, apoptosis, and loss of clonogenic potential. (A, top) Cell cycle analysis. K562 cells stably transfected with either empty vector (pMT; ▧) or a Per2 expression vector (pMTPer2; □) were cultured with zinc for 3 days, harvested, stained with propidium iodide (PI), and analyzed by flow cytometry for cell cycle analysis. indicates wt. (Bottom) Bar graphs present the means ± SD of 3 independent experiments. (B) Apoptosis analysis. Stably transfected K562 cells (pMT and pMTPer2) treated with zinc for 4 days were stained with annexin V-FITC and PI. Data represent the mean ± SD of 3 experiments. (C) Clonogenic analysis. Zinc-treated stably transfected K562 cells (pMT and pMTPer2 [1 × 10⁴/well]) were cultured in soft agar. Colonies containing approximately 1000 cells or more were counted on day 14. Experiments were performed in triplicate and repeated twice, and the mean ± SD of a representative experiment is shown. Untreated wild-type K562 cells (wt) were included as controls.