CMTM4 and head and neck squamous cell carcinoma: The mRNA expression of CMTM4 was found to have a weak positive relationship with PD-L1 in TCGA-LIHC database (rho = 0.116, P = 0.00249, Fig. 1D), which was consistent with recent studies in head and neck squamous cell carcinoma and undifferentiated pleomorphic sarcoma [35, 36].