Additionally, Fibroblast activation protein (FAP), a surface glycoprotein expressed by CAFs, is associated with immune checkpoint molecules (e.g., CTLA4, HAVCR2, and CD276) and contributes to the development and progression of HNSCC via the PI3K-AKT signaling pathway [56,57]. The gene discussed is CTLA4; the disease is head and neck squamous cell carcinoma.