Telomeres are repeating sequences located at each end of eukaryotic chromosomes. These sequences function to protect chromosome positioning and replication (1 –3 ). In vertebrates, telomere DNA consists of tandem repeats of TTAGGG, 10–15 kb pairs long (4 ). In most normal cells, DNA replication during mitosis results in the loss of telomere sequences 50–100 bp at the 5′ ends of DNA termini (1 ,5 ). This sequence loss is mandated by the end-replication-splicing problem (Fig. 1 ). Thus, telomeres progressively shorten with age in somatic cells in culture and in vivo. In contrast, cancer cells and malignant cell lines retain telomere length despite repeated mitosis (6 ). This is believed to be an essential component of immortalization for most cells.
