Glioblastoma (GBM) is a devastating malignant brain tumor with a poor prognosis. GBM is associated with radio-resistance. Post-translational modifications (PTMs) such as protein phosphorylation can play an important role in the cellular response to radiation. To better understand the early cellular activities after radiation in GBM, we carried out a phospho-proteomic study on the U251 cell line, 3h after X-ray irradiation (6Gy) and on non-irradiated cells.