Washington ? Cloning humans, or any other primates, may be impossible with today’s techniques because of a fundamental molecular obstacle, say scientists trying to understand why attempts to clone monkeys have failed.

From the very first step, cloned primate cells don’t divide properly, causing a helter-skelter mix of chromosomes too abnormal for pregnancy to even begin, University of Pittsburgh researchers reported Thursday in the journal Science.

“Most people in the cloning field will be surprised by this,” lead researcher Gerald Schatten said. “This work demonstrates there’s a pothole in the process. We now know the depth and breadth of the pothole, and we’re designing strategies to get around” it.

Dozens of animal clones — including cows, pigs, mice, goats and a cat — have been born since Dolly the sheep became the first new being created from an adult cell in 1997. But it’s still a very uncertain field: Many are stillborn and some survive only with severe defects.

A cult group claimed in December to have cloned a person, something never verified. A doctor who separately is pursuing human cloning has reported in an Internet journal preliminary data on an early-stage cloned human embryo, but with no chromosome information.

Cloning experts worry that attempting human cloning is dangerous not just because of all the barnyard clones with birth defects, but because attempts to clone monkeys — far closer genetically to people — using the Dolly technique have failed.

To clone, scientists harvest an unfertilized egg from a female donor, remove the genetic material and replace it with new DNA from an adult cell of the animal to be cloned. An electric shock coaxes it into dividing. If all goes well, the egg grows into an embryo that can be implanted into a surrogate mother.

It took 277 attempts before Dolly was born. Schatten’s group tried even longer to clone a rhesus monkey — 724 eggs that yielded only 33 embryos and not a single pregnancy.

It’s not just bad news for reproductive cloning. It also means the related field of therapeutic cloning — using embryonic stem cells to grow customized tissues for medical treatment — may prove harder, too, Schatten said. However, if 95 percent of cells growing in a lab dish have abnormal chromosomes, the remaining good 5 percent could still be used, he added.