The genomes of virtually all organisms contain repetitive sequences that are generated by the activity of transposable elements (transposons). This relationship has been largely competitive, and transposons have earned epithets such as ‘junk DNA’ and ‘molecular parasites’. All organisms have evolved diverse mechanisms to repress the activity of their transposons. Recent evidence suggests that retrotransposable elements (retrotranposons) can become activated with age in somatic tissues of diverse species including yeast, Drosophila, and mammals. In particular, during cellular senescence of human cells, LINE-1 retrotransposons become transcriptionally derepressed and activate a type-I inter...