Apoptosis and G2/M arrest in glioblastoma multiforme (GBM) cells is induced by125I seed irradiation. Apoptosis of GBM cells was examined by Annexin V-PI co-staining flow cytometric analysis (A, B) and caspase-3 activity assay (C). (D) Effects of 125I seed on the cell-cycle distribution of GBM cells were examined by flow cytometric analysis. (E) TUNEL assay was used to evaluate the apoptotic response in U251 (left panel) and U87 (right panel) cells. (F) Treatment of cells with 125I seeds caused obvious apoptosis in a dose-dependent manner, accompanied by up-regulation of cleaved caspase-3, cleaved PARP, and phosphorylated Cdc2 (Tyr 15). Data are presented as mean ± SD (n =3). Significant differences between the 125I seed and X-ray groups under the same dose are indicated by *P <0.05 and **P < 0.01.