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Timeline: Fifty Years of Cloning

Highlights in the history of cloning:

1952: Scientists demonstrate they can remove the nucleus from a frog egg, replace it with the nucleus of an embryonic frog cell, and get the egg to develop into a tadpole. This "nuclear transfer" transplants an animal's genes to an egg. The tadpole is a clone of the embryo that donated its nucleus.

1975: Scientists get tadpoles after transferring cell nuclei from adult frogs.

1986: Sheep cloned by nuclear transfer from embryonic cells.

1997: Scientists reveal Dolly the sheep, the first mammal to be cloned from cells of an adult animal.

1998: More than 50 mice reported cloned from a single adult mouse over several generations. Eight calves reported cloned from a single adult cow.

2000: Pigs and goats reported cloned from adult cells.

2001: Advanced Cell Technology of Worcester, Mass., says it produced a six-cell cloned human embryo, in research aimed at harvesting stem cells.

2002: Rabbits and a kitten reported cloned from adult cells.

Dec. 27, 2002: Clonaid claims to produce first human clone, a baby girl.