Each tissue was further separated into six specimens and analysed by both PESI-MS and immunohistochemistry (IHC) for p-SMAD2, a conventional examination to evaluate activation of TGF-β signalling and identify HNSCC cells stimulated with TGF-β within a tumour tissue (Fig. 3b and Supplementary Fig. 3). Here, SMAD2 is linked to head and neck squamous cell carcinoma.