SULF1 and head and neck squamous cell carcinoma: Correlation between SULF2 and SULF1 mRNA is significant (r = 0.56, p = 0.008), similar to the analysis of the TCGA-HNSCC (Supplementary Figure 1A), and stronger than the correlation of other genes; SULF2 showed a similar correlation with EXT2 (r = 0.53, p = 0.014).