PIK3CA and head and neck squamous cell carcinoma: Primary HNSCC gene mutation analysis allowed for the categorization of HNSCCs into distinct groups (basal, mesenchymal, atypical, and classical), comprising cell-cycle regulating genes (CDKN2A and CCND1), genes involved in cell proliferation and survival regulation (TP53, HRAS, PIK3CA, and EGFR), genes responsible for cellular differentiation (NOTCH1), and genes involved in the modulation of adhesion and invasion of a mesenchymal-enriched subtype (Wnt signaling pathway regulator gene FAT1, protocadherin).