The incidence of abnormal gene expression in cloned mammals is more extensive than widely believed, Rudolph Jaenisch and colleagues reported this week in the Proceedings of the National Academy of Sciences. Furthermore, many abnormalities are a consequence of the cloning procedure, whereas others derive from the donor nucleus, said the authors from the Whitehead Institute for Biomedical Research and Massachusetts Institute of Technology.

Cloning mammals involves taking a mature donor nucleus and reprogramming it to a state that supports embryonic development, while suppressing previously transcribed genes involved in cell differentiation in the donor. The majority of cloned mammals die before or soon after birth. The few that do survive, however, are affected by developmental abnormalities to varying degrees.

The extent of these abnormalities was revealed by Jaenisch and colleagues using oligonucleotide microarray analysis, which enabled them to evaluate the expression of more than 10,000 genes in the livers...

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