PIK3CA and esophageal squamous cell carcinoma: Both ESCC and HNSCC share alterations in genes involved in cell cycle regulation (e.g., TP53, RB1, CDKN2A, CCND1 and the PI3K/AKT/mTOR pathway), NOTCH pathway (e.g., NOTCH1) and cellular adhesion (e.g., FAT1).