A good popular science article on telomere biology from Innovations-Reports: "Without the enzyme [called telomerase] that 'regenerates' telomeres (the ends of DNA), stem cells lose functionality and the organism rapidly ages, while it acquires cancer resistance. ... the lack of telomerase causes a severe defect in the fundamental functions of stem cells. ... in genetically modified mice that did not express telomerase, stem cells lost their functionality and became unable to regenerate the damaged epithelial tissue. On the whole, these mice aged more rapidly than normal mice. But, there was a very interesting side effect: without telomerase, mice showed a marked cancer resistance. ... Further experiments on telomeres structure showed that every time the shortening process is altered, the result is either 'early aging and cancer resistance' (if shortening is boosted), or 'aging inhibition and more cancer occurrence' (if shortening is reduced)."