PDCD1 and head and neck squamous cell carcinoma: Soon after, in 2017, after the CheckMate 141 study, and in 2019, after the KEYNOTE-048 study, respectively, nivolumab was approved as a second-line treatment for platinum-resistant recurrent/metastatic HNSCC and pembrolizumab was approved as mono- and combination therapy for localized and recurrent/metastatic HNSCC, and both were checkpoint inhibitors targeting PD-1 [94,97,98,99,100].