PIK3CA and head and neck squamous cell carcinoma: As a good example supporting this, in a deliberate investigation [105] concerning acquired resistance to ICIs, the authors compared the molecular variation in pre- and post-ICI paired tumor samples derived from HNSCC patients with acquired resistance, employing a variety of approaches, including whole-exome sequencing, RNA sequencing and multiplex immunohistochemistry, and revealed that resistant HNSCC tissue harbored an E542K mutation in PIK3CA, which preliminarily seemed to reveal the relationship between the PIK3CA-E542K mutation and acquired resistance to ICIs.