PDCD1 and head and neck cancer: Antibodies that block the PD-1/PD-L1 pathway have been approved for multiple solid tumors, including melanoma [9], lung cancer [10], head and neck cancer [11], urothelial cancer [12], Merkel cell carcinoma [13], and microsatellite instability-high/mismatch repair deficient (MSI-H/dMMR) mCRC.