Telomeres are specialized structures that adorn the ends of human chromosomes. Early studies showed the essential role of telomeres in the integrity of chromosomes35–37. These nucleoprotein caps are maintained by the enzyme telomerase38–40. The importance of ad equate telomerase activity and maintenance of telomere length for both replicative potential in culture and ageing in organisms was initially inferred from studies of primary human fibroblasts. In culture, division of fibro-blasts results in progressive telomere attrition, culminating in a state of proliferative arrest — or cellular senescence — after a finite number of cell divisions, a barrier known as the Hayflick limit41. Proliferation beyond this limit drives further telomere erosion, ultimately triggering rampant chromosomal instability driven by chromosome breakage–fusion–breakage events42.
