HDAC inhibitors (HDACi) suppress the growth of tumor cells due to induction of cell cycle arrest, senescence or apoptosis. Recent data demonstrate that HDACi can interfere with DNA Damage Response (DDR) thereby sensitizing the cells to DNA damaging agents. Here, we show that HDACi sodium butyrate (NaBut) potentiates the formation of γH2AX foci predominantly in S-phase E1A+Ras cells. Accumulation of γH2AX foci sensitizes the cells toward such DNA damaging agents as irradiation (IR) and adriamycin. In fact, NaBut potentiates the persistence of γH2AX foci induced by genotoxic agents. The synergizing effects depend on DNA damaging factors and on the order of NaBut treatment. Indeed, NaBut treatment for 24 h leads to an accumulation of G 1-phase cells and a lack of S-phase cells, therefore, adriamycin, a powerful S-phase-specific inhibitor, when added to NaBut-treated cells, is unable to substantially add γH2AX foci. In contrast, IR produces both single- and double-strand DNA breaks at any stage of the cell cycle and was shown to increase γH2AX foci in NaBut-treated cells. Further, a lifetime of IR-induced γH2AX foci depends on the subsequent presence of HDACi. Correspondingly, NaBut withdrawal leads to the extinction of IR-induced γH2AX foci. This necessitates HDACi to hold the IR-induced γH2AX foci unrepaired. However, the IR-induced γH2AX foci persist after long-term NaBut treatment (72 h) even after washing the drug. Thus, although signaling pathways regulating H2AX phosphorylation in NaBut-treated cells remain to be investigated, the obtained results show that NaBut potentiates effects of DNA damaging agents by facilitating formation and persistence of γH2AX foci.