SMAD3 and head and neck squamous cell carcinoma: To investigate whether palbociclib treatment elicited EMT through TGF-β/Smad3 signaling in HNSCC, CAL27, FADU, and HN30 cells were next incubated with TGF-β1, SB-431542 (a TGF-β type I receptor kinase inhibitor) and palbociclib in the absence or presence of SB-431542.