CD274 and head and neck squamous cell carcinoma: PD-L1 immunohistochemistry (IHC) has received regulatory approval as a companion or complementary diagnostic test for several ICIs across various tumor types, including non-small cell lung cancer (NSCLC), gastric cancer, triple-negative breast cancer (TNBC), cervical cancer, urothelial carcinoma, and head and neck squamous cell carcinoma (HNSCC) (21).