Murine embryonic fibroblasts were left untreated (top) or incubated with bleomycin (bottom). Bleomycin-induced cellular senescence is demonstrated by the enlarged cell morphology and stain ing for histone H3K9 trimethylation (green nuclear foci), which was superimposed to staining for α-tubulin (red) and DNA (blue). For more information, see Hildebrand et al., pp. 1922–7.