EGFR and head and neck squamous cell carcinoma: In the context of HNSCC, the ligand-dependent mode is often observed, where the availability of EGFR ligands is the rate-limiting factor where the abundance of EGFR ligands (AREG, EREG, HBEGF, EPGN and TGFA), are associated with EGFR pathway activation and response towards cetuximab in the patient-derived xenograft (PDX) models20.