Telomeres are the ends of chromosomes. They consist of repetitive sequences that vary in length between species and individuals. With each cell division, telomeres shorten due to oxidative stress and the inability of DNA polymerase to fully replicate them (end replication problem). Several means to lengthen telomeres have been observed; in humans, two methods exist: the telomerase enzyme and the recombination-based alternative lengthening of telomeres (ALT). Telomeres play a significant role in cancer development and stem cell aging.
90% of cancers activate telomerase as a telomere maintenance mechanism.