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explainer: Answers to your questions about the news.

Why Is Cloning Humans So Hard?Are we really that different from sheep or cows or pigs?

By Michelle Tsai
Posted Monday, Nov. 12, 2007, at 7:19 PM ET

Illustration by Mark Alan Stamaty. Click image to expand.

The Telegraph reported on Sunday that a research team has produced dozens of embryos cloned from an adult monkey, something that had never before been accomplished. We've already cloned many other kinds of animals, like sheep, cats, and mules, but the most recent breakthrough suggests that human cloning might be just around the corner. So, what makes cloning people so hard?

The process of finding human eggs. To clone an animal, scientists need two cells: an egg and a donor cell. Scientists remove the nucleus from the egg and replace it with the one from the donor cell. For the animals we know how to clone, it can take 100 or more tries—and just as many egg cells—to complete the procedure. To develop the technology for a new species, like humans, there could be even more trial and error. That's not a problem if egg cells are easy to find. Scientists can procure cow ovaries, for instance, from slaughterhouses: A bucket of 100 ovaries can yield 1,000 eggs. To get human cells, they'd have to find women who are willing to undergo about a month of hormone treatments and then surgery to extract the eggs. Sure, women who are egg donors already do this, but it's not volunteer work—compensation can go as high as $35,000. So far, researchers haven't had enough human egg cells on hand to produce a successful clone.

There might be some other kinks that could make human cloning more difficult, but no one knows for sure. (The South Korean lab that reported the creation of 11 embryo lines in 2005 fabricated much of their data.) Each animal poses its own specific problems that must be overcome: At first, pigs couldn't be cloned because their fatty egg cells appeared black under the microscope, and scientists couldn't locate the nuclei that needed to be removed. (Now there's a way to stain the nucleus and illuminate them with ultraviolet light.) The yolks of bird eggs also make it impossible to spot the nucleus, which is why no birds have ever been cloned. Dogs are especially tricky to clone because their eggs mature in the oviduct, not in the ovary, and scientists haven't figured out how to bring the eggs to maturity in the lab. Success rates can be as high as 50 percent for cows, but just 1 percent to 3 percent for mice.

Got a question about today's news? Ask the Explainer.

Explainer thanks Duane Kraemer of Texas A&M University, Alex Meissner of the Whitehead Institute, James Robl of Hematech, and Dirk Vanderwall of University of Idaho.