Telomerase is an enzyme that is responsible for maintaining a balance between telomere shortening and elongation by adding telomeric DNA repeat sequences to the ends of chromosomes. Except for neurons and heart myocytes, all cells in the body age and die long before the body dies; therefore, cells must periodically replicate and replace dying cells. Most somatic cells do not express telomerase and therefore lose telomere length with age, leading to death of the cell. However, cancer cells are immortal because telomerase is expressed in cancer cells. This makes telomerase an attractive target for cancer diagnosis and therapy.