Expression of Stat1Ser727Ala in U-937 cells inhibits ATRA-induced growth arrest. (A) pCIneo, Stat1.2, Ser727Ala.4, and Ser727Ala.5 cells were seeded at 100 000 cells/mL in the presence of ATRA and counted daily. Results are shown as the percentage of untreated control cells. Mean ± SD (n = 3). (B) Cells were incubated with ATRA for 48 hours, and DNA synthesis was determined by measuring [³H]-thymidine (TdR) incorporation correlated to the total cell number. Data for incorporated [³H]-TdR per cell are shown. Mean ± SD (n = 3). (C) The distribution of cells in different cell cycle phases was determined by flow cytometry of propidium iodide—stained nuclei after 72 and 96 hours of ATRA induction. The graph shows the percentage of cells in the S + G₂/M phases. Mean ± SD (n = 3). Representative cell cycle profiles (72 hours after induction) are shown in the right histograms.