Several teams have tried to make animal-human hybrids as a source of embryonic stem cells, the master cells of the body. Because human eggs are scarce -- it requires a surgical procedure to get them from a woman -- some scientists came up with the idea of using animal egg cells.

The cloning technique is called somatic cell nuclear transfer. The nucleus is removed from an egg cell and replaced with the nucleus from another type of cell from the donor animal or person who is to be cloned.

Done right, the process starts the egg growing and dividing as if it had been fertilized by a sperm, but the resulting embryo carries mostly the DNA of the donor.

"The idea was to simply to plunk a patient's DNA into an empty cow or rabbit egg -- and presto -- you reprogram the DNA back into a stem cell," Lanza said in a telephone interview.

But teams that have tried to do this have always ended up with what looks like a cell dividing over and over to become an embryo, but which eventually fizzles out.

"For the last decade, we've carried out literally hundreds of experiments trying to create patient-specific stem cells using animal eggs," Lanza said.

A mouse-human hybrid petered out after just one division. The cow and rabbit human hybrids went further, but stopped at the point when maternal DNA is supposed to kick in and turn the ball of cells into a proper embryo, Lanza said.