Short telomeres reduce the risk of cancer early in life at the expense of impaired regeneration late in life. Genetic variation in telomere length between humans is shown in blue. The TEDS theory of aging proposes that telomere erosion allowed lifespan to increase by suppressing the growth of malignant tumors before reproduction. Telomere loss has pleiotropic detrimental effects late in life by limiting cell renewal in the immune system and other tissues. Long and short telomeres increase the risk of cancer late in life via distinct, but partially overlapping, mechanisms (see text for details). Note that the rate of telomere loss depends, not only on cell divisions, but also on damage to telomeric DNA and variable levels of telomerase. The replication potential or maximum number of cell divisions (n) in stem cells is not known but is predicted to be less than 100 by TEDS: 50 to 60 divisions predicted by Schrödinger47 plus 0 to 40 additional cell divisions to account for stem cell renewal over a lifetime.