TGFB1 and head and neck squamous cell carcinoma: Exposing cultured HNSCC keratinocytes to 5 ng/ml TGF‐β1 for 48 h and measuring the levels of phosphorylated SMAD3 identified two populations that were not responsive to TGF‐β (HN9 and HN20) and a range of stimulation (2‐ to 4‐fold) in the remaining four populations (Figure 2A).