BRISTOL Although nuclear transfer technology has been used to produce live clones in several species including cattle, sheep, goats, pigs and mice, only a small percentage of nuclear transfer embryos develop to term. A team from the Whitehead Institute for Biomedical Research, Cambridge, Massachusetts, and the Department of Anatomy and Reproductive Biology, University of Hawaii, analyzed imprinted gene expression in mice cloned by nuclear transfer and compared it to expression in the original embryonic stem (ES) cell population. The aim was to try to correlate gene expression with both survival and foetal overgrowth.

They found that the epigenetic state of the ES cell genome was extremely unstable and that variation in imprinted gene expression occurred in most cloned mice - even in those derived from ES cells of the same clone. But the most intriguing finding was that a small number of the mice survived to adulthood despite...

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