Background: Colorectal cancer (CRC) is characterized by extensive DNA methylation at the promoters of tumor suppressor genes and enhanced activity of histone modifying proteins such as histone acetyltransferases (HATs) or histone readers such as bromodomain and extra-terminal domain (BET) proteins. BET inhibition has been associated with the reduced expression of DNA repair proteins and therefore can be used in combination with radiotherapy (RT) and chemotherapy. In the current study, we have evaluated the therapeutic potential of a clinical stage (phase 1) dual BET and CBP/p300 inhibitor, EP31670 (EP), in combination with RT. Methods: HCT116 CRC 2-D cultures were treated with EP 4 hours before RT. After pretreatment with EP (0, 0.1, 0.5, 1, 2, 5, and 10µM), cells were irradiated with various doses (0, 2, 4, 6, and 8Gy) of RT (300-kv x-rays). Crystal violet (CV) assay at 48 hours post RT was used to determine growth inhibition and viability between the various drug, RT, and combination groups. Synergy between treatments was evaluated by estimating the combination index (CI) values via mass-action law CompuSyn calculations using CV data. CI values of <1 from the growth inhibition assay suggested that EP synergized with RT for enhanced cytotoxicity in HCT116 CRC cells. Flow cytometry was used to assess DNA damage (γH2Ax), apoptosis and total death (Annexin V-PI assay), cell cycle analysis, and proliferation (EdU). Results: There was a significant dose-dependent increase in the residual DNA damage at 6 hours (8Gy + EP 0.5µM vs. 8Gy, p<0.0001) and 24 hours (8Gy + EP 0.5µM vs. 8Gy, p=0.002) for combination therapy over RT alone. Four-hour treatment with EP promoted G2 cell-cycle arrest (8Gy + EP 0.5µM vs. 8Gy, p<0.001) and significantly reduced the number of HCT cells in S-phase (8Gy + EP 0.5µM vs. 8Gy, p<0.001) and proliferation (EdU) of the HCT cells (8Gy + EP 0.5µM vs. 8Gy, p=0.0003) compared to RT alone. There was a significant increase in apoptotic cells the combination group (8Gy + EP 0.5µM vs. EP 0.5µM, p=0.008) when compared to the unirradiated cells. The combination group also showed significant increase in the total death compared to RT alone (8Gy + EP 0.5µM vs. 8Gy, p=0.03) at the G1-S checkpoint. Conclusions: EP31670 enhanced RT-induced growth arrest, reduced proliferation, and enhanced cell death in HCT116 CRC cells in synergy with the RT. Additional preclinical studies, in vivo studies, and eventual clinical studies can be conducted to further exploit this synergistic interaction.