Origins of senescence-associated γ-foci. (A) Distribution of γ-foci in high population doubling (PD) WI-38 cells, after the expression of telomerase (TERT), after culturing with the 50 μM antioxidant tempol (OX), or in VA-13 with alternative lengthening of telomeres (ALT). (B) Distribution of γ-foci in high PD mouse embryonic fibroblast (MEF) cells, in spontaneously immortalized MEF cells (IM) or after culturing with the 50 μM tempol (OX). Scoring is as in Figure 1B. On average more than 10 metaphases were screened per point in independent experiments. Error bars signify standard errors.