SERPINE1 and neoplasm: In order to identify senescent tumor cells at single-cells resolution, we defined a senescence signature by combining the expression of p16INK4a, p15INK4b, p19Arf, p21Waf1/Cip1, p27Kip1, and PAI-1, the most upregulated senescent markers in whole tumors lysates, as detected by western blot analysis1,21 (Fig. 1c, Supplementary Fig. 1a).