Genomic landscape of Polycomb orthologs is preserved at senescence. (A) Staining for senescence-associated β-galactosidase activity in proliferating (Pro) and senescent (Sen) Hs68 cultures. (B) Immunoblotting for p16INK4a, p18INK4c, CBX7 and β-tubulin in proliferating and senescent cells. The asterisk identifies a non-specific band detected with the CBX7 antibody. (C) Examples of ChIP-sequencing data obtained for proliferating or senescent Hs68 cells with antibodies against CBX7 and CBX8. Note that the sequence tag densities are not plotted on equivalent scales. kb, kilobase; Pro, proliferating; Sen, senescent.