Telomerase is the reverse transcriptase responsible for the extension of telomeric repeat sequences in most species studied. If telomerase activity is diminished or absent, telomeres will shorten. Shortened telomeres appear to lead to cell senescence. Eventually telomeric sequences can shorten to the point where they are not long enough to support the telomere-protein complex protecting the ends and the chromosomes become unstable. These shortened ends become 'sticky' and promote chromosome rearrangements. Some rearrangements may contribute to the development of cancers.