Human cloning may never be possible because of a quirk of biology.
Scientists in the United States say hundreds of attempts to clone monkeys have ended in failure.
Something goes wrong as the cell divides (Science)
They think the biological make-up of the eggs of primates, including humans, makes cloning almost impossible.
Cloning has been successful in several mammals, including sheep, mice and cattle, but there is increasing evidence that it does not work in all species.
The research, reported in the journal Science, casts further doubt on efforts by a handful of mavericks to clone humans.
Clonaid, a company created by a UFO cult known as the Raelians, claims to have already cloned several babies. It has produced no evidence to substantiate these claims.
Meanwhile, controversial reproductive scientist Panayiotis Zavos has published a picture of what he claims is "the first human cloned embryo for reproductive purposes".
The majority of scientists agree that attempts to clone a human baby are dangerous and misguided.
Many cloned animals have been born ill or deformed and successful births are few and far between.
Researchers at the University of Pittsburgh School of Medicine used the method pioneered on Dolly the sheep to try to clone rhesus macaque monkeys.
They were unable to establish a single pregnancy after hundreds of attempts. Other groups have also tried and failed to clone monkeys.
The obstacle appears to be something to do with the way genetic material is parcelled up as a cell splits into two during embryonic development. Cells end up with too much, or too little DNA, and cannot survive.
It suggests that attempts to clone other primates, even humans, may be doomed to failure.
"This reinforces the fact that the charlatans who claim to have cloned humans have never understood enough cell or developmental biology (to succeed)," team leader Dr Gerald Schatten told the journal Science.