Senescence as a general stress-response program. A variety of physiologic stresses, intrinsic and extrinsic, lead to the onset of senescence. These stresses stimulate various cellular signaling pathways, which are funneled down to activate either the p53 protein, the Rb protein, or both. p53 can be activated by the DNA damage signaling pathways, the ATM and ATR proteins, or by the p14/ARF protein, which responds to oncogene overexpression and other stresses. p21, a target of p53, can cause the activation of Rb. Most cellular stresses will activate the p16/INK4a gene, also leading to Rb activation. Different stress signals will have different relative effects on the p53 and Rb arms, and their combined level of activation dictates the onset of senescence. Once this program is activated, a series of changes in cell function and morphology take place. ATM, ataxia telangiectasia mutated; ATR, ATM-related; p14/ARF, alternative reading frame product of INK4a gene locus.