A single genetic difference may make the cloning process less complicated in humans than in sheep and other mammals, scientists say. According to a report published today in the journal Human Molecular Genetics, humans and their primate kin possess two active copies of a gene called insulin-like growth factor II receptor (IGF2R) that prevents fetal overgrowth¿a problem that has often thwarted efforts to clone animals. Nonprimates have only one functional copy of this gene, the result of a phenomenon called genomic imprinting, which effectively silences the other copy. As a result, these animals are more susceptible to cancer and to cloning-related complications.

Keith Killian and his colleagues at Duke University used distinctive genetic markers known as single nucleotide polymorphisms (SNPs) to test for imprinted IGF2R in humans and to reconstruct the evolutionary history of the imprinted gene. The team found no evidence of it in humans. Indeed, the results indicate that primates and their closest relatives lost the imprinted gene some 70 million years ago. In contrast, other mammals¿including mice, sheep, cows, pigs and opossums¿still carry it.

"This is the first concrete genetic data showing that the cloning process could be less complicated in humans than in sheep," Killian observes. "Only one in 300 sheep embryos takes hold, and up to half of these embryos have large offspring syndrome, which can kill the mother and fetus. Since humans are not imprinted at IGF2R, fetal overgrowth would not be predicted to occur if humans were cloned."

Other scientists assert that the study authors' conclusions are flawed, noting that other genes may contribute to the problems seen in cloning sheep. "It seems that a little knowledge is a dangerous thing, and the authors have allowed themselves to overinterpret their interesting results," Ian Wilmut, who created the cloned sheep Dolly, told Reuters. "I hope that this will not be used to give encouragement to those who wish to clone humans."

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