1938
Cloning is envisioned by German scientist Hans Spemann. He theorizes that animals could be cloned by fusing an embryo (early developing animal) with an egg cell.
1952
Robert Briggs and T.J. King use a pipette to suck the nucleus from the cell of an advanced frog embryo and add it to a frog egg. It did not develop.
1962
British molecular biologist John B. Gurdon announces he has cloned frogs using the nuclei of fully differentiated adult intestinal cells.
1970
Gurdon tries the same procedure again. He transplants frog embryo cells into egg cells. The eggs developed into tadpoles but died after they were ready to begin feeding. He later showed that transplanted nuclei reverted to an embryonic state.
1977
Gurdon's image of 30 cloned frogs sparks public coverage that associates cloning research with 'Brave New World'.
1978
In England, Louise Brown is born -- the first baby produced after in vitro fertilization.
The release of David Rorvik's book, "In His Image: The Cloning of a Man," sparks a worldwide debate on cloning ethics.
1981
Karl Illmensee of the University of Geneva and Peter Hoppe of the Jackson Laboratory in Bar Harbor, Maine claim to have cloned normal mice from mouse embryo cells.
1982
Drs. James McGrath and Davor Stolter of the Wistar Institute of Anatomy and Biology in Philadelphia report they could not repeat the mouse-cloning experiment and conclude that once mouse embryos have reached the two-cell stage they cannot be used for cloning. Others confirm their results.
1984
Dr. Steen Willadsen at the Institute of Animal Physiology in Cambridge, England reports that he has cloned a live lamb from immature sheep embryo cells. Others later replicate his experiment using a variety of animals, including cattle, pigs, goats, rabbits and rhesus monkeys.
1994
Dr. Neal First of the University of Wisconsin cloned calves from embryos that have grown to at least 120 cells.
1996
Dr. Ian Wilmut repeats Dr. First's experiment with sheep, but puts embryo cells into a resting state before transferring their nuclei to sheep eggs. Then he fuses them with egg cells. The cells begin to divide like new embryo cells.
1997
Dr. Wilmut reports that he has cloned a 6-year-old adult sheep from an udder cell, the world's first creature to be cloned from adult cells.
1998
Researchers at Japan's Kinki University say they have cloned eight calves from a single cow, but only four survived to their first birthday.
The U.S. Food and Drug Administration declares authority to regulate human cloning.
1999
Medical researchers at Kyunghee University Hospital in Seoul succeed in cloning a human cell from an infertile woman, creating a four-celled embryo that theoretically could have grown into a genetically identical replica of the woman. Because of legal and ethical implications of their work, they end the experiment without implanting the embryo in a woman.
2000
A female rhesus monkey named Tetra is cloned, the closest human genetic relative cloned successfully. Although mice and cows have been reproduced, this is the first time for a monkey.
Teams of researchers in Japan and Scotland announce they have cloned pigs. The use of pig tissue is likely to be the most compatible with humans to grow replacement hearts and other organs.
