Senescent cells show a series of alterations, including a flat and enlarged morphology, increase in nonspecific acidic β- galactosidase activity, chromatin condensation, and changes in gene expression patterns. The onset and maintenance of senescence are regulated by two tumor suppressor proteins, p53 and Rb, whose expression is controlled by two distinct proteins, p19(Arf) and p16(Ink4a), respectively, which are encoded by the cdkn2a locus. Transcription factor Jun dimerization protein 2 (JDP2) which binds directly to histones and DNA, inhibits the acetylation and methylation of core histones and of reconstituted nucleosomes that contain JDP2-recognition DNA sequences. JDP2-deficient mouse embryonic fibroblasts are known to be resistant to replicative senescence. Oxygen induces the expression of the JDP2 gene and JDP2 then inhibits the recruitment of polycomb repressive complexes (PRCs1 and 2) to the promoter of the gene encoding p16(Ink4a), resulting in the inhibition of methylation of lysine 27 of histone H3. These findings suggest that chromatin-remodeling factors, including the PRC complex controlled by JDP2, are important players in the senescence. The newly defined mechanisms that underlie the action of oxygen in the induction of JDP2 and cellular senescence are reviewed.