HDAC9 and head and neck squamous cell carcinoma: In the training cohort, using univariate Cox regression analysis and LASSO regression analysis, seven mRNAs were identified as prognostic factors in patients with HNSCC (ZNF439 (encoding zinc finger protein 439), KMO (encoding kynurenine 3-monooxygenase), KDM5D (encoding lysine demethylase 5D), IFT57 (encoding intraflagellar transport 57), HDAC9 (encoding histone deacetylase 9), GSAP (encoding gamma-secretase activating protein), and CCR7 (encoding C–C motif chemokine receptor 7); Figure 3A,B).