In our results, we found that mRNA expression of PD-1 (PDCD1) was evidently higher in lung adenocarcinoma/squamous cell carcinoma, hepatocellular carcinoma, esophageal carcinoma, head and neck squamous cell carcinoma, breast invasive carcinoma and renal clear cell carcinoma than that in normal control (Figure 2; all P<0.05). Here, PDCD1 is linked to head and neck squamous cell carcinoma.