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Cellula r senescence is a general stress-response program that restrains cellular proliferation.We recently demonstratedthat telomere-initiated cellular senescence is a DNA-damage response enforced by the activation of the DNA-damage checkpoint machinery with the direct contribution of eroded telomeres.We will provide evidence that the DNA-damage response observed in senescent cells is not a transient phenomenon,but it consists of a permanent activation of the DNA damage checkpoint machinery,that,if left unperturbed,persists presumably ad infini tum.We propose that this is due to the DNA damage being irreparable in senescent cells.Its irreparability is not due to the inability of senescent cells to repair exogenous DNA damage but to the intrinsic nature of their DNA lesions.