HDAC9 and head and neck squamous cell carcinoma: As previously mentioned, CUDC-101 is both a multi-targeted EGFR and HDAC inhibitor; however, given that EGFR targeting in HNSCC clinically has failed [34,35], combined with our observations that mocetinostat and pracinostat (and possibly other HDAC inhibitors included in the spheroid screen) are equally effective, if not more, in radiosensitising HNSCC cells in vitro, then we would advocate that HDAC inhibition is the key driver and should be a strategy for further investigation.