Despite some success, reproductive cloning in mammals is still a tricky feat. University of Pennsylvania School of Veterinary Medicine researchers, by tracking the gene Oct4 in mice, have shown how its routine failure to reprogram after nuclear transplant commonly prevents the successful development of mammalian embryos (M. Boiani et al., "Oct4 distribution and level in mouse clones: consequences for pluripotency," Genes & Development, 6[10]:1209-19, May 15, 2002). Producing a clone requires that the donor nucleus abort its current genetic program and express that of an embryonic nucleus. Oct4, which is not expressed in adult somatic cells, encodes a transcription factor essential to embryonic development and viability. "Without its expression," says Hans Schöler, director of Penn's Center for Animal Transgenesis and Germ Cell Research, "you can't set aside the pluripotent stem cells." Schöler and colleagues tracked Oct4 expression in the inner cell mass (ICM), the embryonic stem...

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