Ape1 regulates cell cycle G1-S transition in day-3 EB cells. Day-3 EB cells or Flk-1⁺ day-3 EB cells were treated with either transfection reagent (Oligofectamine 2000) alone, scrambled siRNA (25 nM), or Ape1 siRNA (25 nM) for 48 hours. Cells were collected, permeablized, and stained with PI prior to flow cytometry to determine the percentage of cells in G1 and S phase. Open bars represent day-3 EB cells and solid black bars represent Flk-1⁺ day-3 EB cells. (A) Percentage of G1 phase cells. (B) Percentage of S phase cells. (A-B) Bars represent means ± SD of triplicate experiments. *P < .05.