Q-FISH measurements of metaphase chromosomes from human fibroblasts at different population doublings (PDs). The telomere length values are increasingly skewed toward short telomeres as the cultured cells approach senescence. (A) Actual data (bars; adapted from Martens et al, with permission²⁸) correspond to predictions based on a mathematical model (lines; adapted from Rodriguez-Brenes and Peskin with permission⁹⁹) but show more pronounced skewing. (B) Snapshots of Q-FISH images of chromosomes X and 17 (also hybridized with a Chr17-specific probe) from individual metaphase cells of different diploid fibroblasts clones. For details see Martens et al,²⁸ Asterisks indicate sporadic loss of telomere repeats on specific chromosome arms. Q-FISH, quantitative fluorescence in situ hybridization.