PIK3CA and head and neck squamous cell carcinoma: Several pan-PI3K (e.g. Copanlisib, BKM-120, PX-866) and isoform-selective PI3K (e.g. Alpelisib, CYH33, AZD8186, Idelalisib) inhibitors elicit significant anti-tumor activity in HNSCC and other tumor entities [3,4].