Current animal cloning techniques require that a single diploid nucleus be introduced into an enucleated oocyte, which is then stimulated to restart the embryonic development program. Recent evidence suggests that the source of the nuclei governs the success of the technique, such that nuclei that have a lower level of epigenetic imprinting are more likely to achieve a successful clone because this low level is easier to erase, aiding the restart process. In the May 2 Sciencexpress, Karin Hübner and colleagues at the Center for Animal Transgenesis and Germ Cell Research, University of Pennsylvania, directly derive blastocysts from embryonic stem (ES) cells without nuclear transfer (Sciencexpress, DOI:10.1126/science.1083452, 1 May, 2003).

Hübner et al. built on their previous finding that only one of two different enhancers that govern expression of the Oct-4 gene, a gene controlling pluripotency in embryos and germ cells, is used depending on whether...

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