The ends of chromosomes are specialized structures called telomeres. These structures are essential for chromosomal stability. The enzyme telomerase is responsible for maintaining the telomeres. In human embryos, nearly all cells have telomerase activity, but, with few exceptions, normal adult human cells have lost their telomerase. Cancer cells, however, are rapidly dividing and immortal; that is, they continue to divide even when noncancerous cells of the same tissue have lost that ability. The reactivation of telomerase may be a feature of most human tumors. In this module you will explore how scientists are using this feature of cancer cells as a diagnostic tool and perhaps even as a treatment for human cancer.