PIK3CA and head and neck squamous cell carcinoma: Various therapeutic biomarkers, ranging from viral oncoproteins (e.g., HPV, EBV) and receptor tyrosine kinases (e.g., EGFR, PIK3CA) to immune checkpoint markers (e.g., PD-L1, PD-L2) and tumor suppressor proteins (e.g., TP53) have been identified as therapeutic targets in HNSCC [142].