As judged by immunofluorescence imaging and fluorescence-activated cell sorting (FACS), TGFβ signalling and the phosphorylation of its downstream target transcriptional cofactor SMAD2 (pSMAD2) were rare in papillomas but increased substantially in invasive SCC progenitors (Fig. 1b and Extended Data Fig. 1a,b). Here, TGFB1 is linked to papilloma.