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As a checkpoint protein, p53 is activated by various stressful signals via a disruption of a p53/Mdm2 feedback loop including destruction of MdmX protein. p53 activation is accompanied with protein stabilization and increased transcrip-tional activity via many types of posttranslational modifications including phosphorylation, acetylation, sumoylation, ubiquitina -tion, deubiquitination and methylation.