Carolyn Widney "Carol" Greider (born April 15, 1961) is an American molecular biologist. She is Daniel Nathans Professor and Director of Molecular Biology and Genetics at Johns Hopkins University. She discovered the enzyme telomerase in 1984, when she was a graduate student of Elizabeth Blackburn at the University of California, Berkeley. Greider pioneered research on the structure of telomeres, the ends of the chromosomes. She was awarded the 2009 Nobel Prize for Physiology or Medicine, along with Blackburn and Jack W. Szostak, for their discovery that telomeres are protected from progressive shortening by the enzyme telomerase.