Collectively, this provides evidence that that the class I HDAC inhibitor mocetinostat, the pan-HDAC inhibitor pracinostat, and the broad-spectrum HDAC–EGFR inhibitor CUDC-101 induced radiosensitisation in HNSCC cells through their impact in suppressing the efficiency of DSB repair. The gene discussed is HDAC9; the disease is head and neck squamous cell carcinoma.