PKD1 and head and neck squamous cell carcinoma: Additionally, analysis of the mRNA expression data from the TCGA cohort via cBioPortal demonstrated low mRNA expression in most tumor tissues (Additional file 1: Figure S1A), and PKD1 mRNA was downregulated in 87% HNSCC tumors (459 out of 530 cases with over 2-fold mRNA downregulation), which is consistent with our findings.