Cell cycle analysis further confirmed that more PC-3 and LNCaP cells were blocked in G2/M phase after 24 h LBH589 Nutlin-3 treatment and the percentage of cells in G1 phase decreased significantly. Moreover, treatment with LBH589 for 24 h reduced the S-phase content of LNCaP cells to a lower level. In all the cell cycle phases, G2/M phase cells are the most sensitive to radiation and S phase cells are the most resistant. Our results suggest that cell cycle arrest at G2/M and decrease of S phase percentage might be responsible for the radiosensitization effect of LBH589. The p53-p21 axis plays a very important role in the regulation of cell cycle in CaP RT. The check point proteins Chk1/2-mediated p53 phosphoralation can result in the activation of p21 transcription, thereby inhibits Cdk activity and leads to cell cycle arrest at G1 phase. It was reported that up-regulation of p21 and its subsequent binding to the CDK1-cyclin complexes inhibit CDK1 phosphorylation and lead to a G2/M cell cycle arrest effect. Here, we found that in the later time points, both p53 and p21 proteins were lower in the combination treatment compared to those treated with RT alone, which is in accordance with the G1 defect in cell cycle following the combination treatment in the later time points. The initial increase of p21, p53 and reduction of p-CDK1 after LBH589 treatment especially in LNCaP cells indicate a potential mechanism for G2/M arrest caused by LBH589. The deficiency of p53 in PC-3 cells suggests that p21 may be regulated by alternative mechanisms. After radiation, G2/M arrest is a protective reaction which enables cells to repair DNA damage before entering Paclitaxel mitosis. Correspondingly two cell cycle checkpoints, p-Rb795 and p-Rb807/811, were activated after 2 Gy RT, but combination treatment significantly reduced all their activation in both PC-3 and LNCaP cells, indicating LBH589 treatment at IC20 concentrations were effective to perturb CaP cells�� regulation of cell cycle after RT. These results may also explain part of the radiosensitization effect of LBH589. In our study, expression of ��H2AX was enhanced by combination treatment with LBH589 and RT in both PC-3 and LNCaP cells, including LBH589-induced endogenous foci before radiation as well as promoting the radiation-induced foci. NHEJ and HR pathways are the most important two signaling pathways responsible for repairing DNA DSBs. We found for the first time that key proteins including Ku70 and Ku80 are activated in PC-3 and LNCaP cells after RT and that both Ku70 and Ku80 proteins were less activated in CaP cells by LBH589 pretreatment, which implies that the NHEJ repair pathway plays an important role in the regulation of CaP radiosensitivity after exposure to RT. HR pathway repairs DSB using a homologous chromatid or chromosome.