HDAC4 and head and neck squamous cell carcinoma: Next, we evaluated the influence of HDAC4 and HDAC5 overexpression on the cytotoxic effect of the class IIa-selective HDACi CDHI0039, the pan-HDACi vorinostat, and the proteasome inhibitor bortezomib, as well as cisplatin, which is the standard of care for HNSCC.