Awarded today to Elizabeth Blackburn, Carol Greider and Jack Szostak, for their discovery of telomeres and the enzyme telomerase. Human chromosomes are long linear strands of DNA, which present problems regarding degradation from the ends. Telomeres are special sequences of DNA at the ends of the strands which protect the DNA from degradation and aging of the cell. The enzyme telomerase rebuilds these sequences at the ends of the chromosomes, thereby maintaining a cell's viability. Cells that lose their telomerase activity show decreased growth, e.g., bone marrow stem cells in aplastic anemia, while cells with high levels of telomerase grow rapidly, e.g., cancer cells and embryonic stem cells.