Knocking down Zfx in human malignant cell line U251 arrested the cell cycle. Knockdown of Zfx expression induced S arrest in U251 cells. (A) Cell cycle of U251 cells was analyzed by flow cytometry. (B) S cell cycle phase determined by flow cytometry. Compared with NC, Zfx-siRNA cultures showed a significant increase in cells in S (P = 0.003; P < 0.05), compared with NC. (C) Percentage of apoptosis was plotted against U251 cell line. There was a greater amount of apoptosis in the Zfx down-regulated group of human brain glioma U251 cells (P = 0.0009, P < 0.05). The assay showed a marked induction of apoptosis with 5.51% apoptotic for NC group.