SMAD4 and head and neck squamous cell carcinoma: The single‐gene knockout model of SMAD4 in head and neck epithelia (HN‐Smad4del/del) published by Bornstein and colleagues in 2009 may be the GEMM of spontaneous HNSCC most comparable to the molecular characteristics of clinical disease and has been used to analyze in depth the molecular pathways underlying HNSCC development.82