PPL Therapeutics Ltd of Roslin, Edinburgh, showed today how they succeeded a year ago (the delay was to secure a patent) in targeting a gene to a chosen location in embryonic sheep fibroblasts. Then they transferred the nucleus to stem cells and created whole transformed, gene-targeted sheep (Nature paper 1 and paper 2).

We have succeeded in cloning sheep, goats, cows, pigs and mice, and now we have gene targeting in sheep. But it is proving difficult to clone whole animals efficiently, as the science involved is obscure (see Science vol 288, 9 June 2000, p 1722). According to Science "out of some 100 attempts to clone an animal, typically just two or three live offspring result. Even when an embryo does successfully implant in the womb, pregnancies often end in miscarriage. A significant fraction of the animals that are born die shortly after birth. And some of...

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