Recently, when analyzing the expression patterns of PD-1 and ICOS on CD4+ Th TILs in HNSCC and CRC tissues, researchers found that PD-1+ICOS+CD4+ Th TILs exhibited a tissue-resident memory phenotype with an oligoclonal expansion of their TCR repertoire, which occurred in tumors but was present at a low frequency in the periphery. This evidence concerns the gene PDCD1 and head and neck squamous cell carcinoma.