SERPINE1 and adenoma: Using this fibroblast model system followed by first a learning and then a validation set of adenoma samples, we generated a senescence surrogate score (SI) based on the expression of p-ERK, HP1γ and PAI-1 – all of them previously linked to OIS (Serrano et al, 1997; Lin et al, 1998; Narita et al, 2003) – that recapitulated SA-β-gal reactivity and adenoma senescence (Figure 1C; Figure 2).