By cloning embryos using nuclear DNA from sick people, the scientists aim to produce stem cells that are genetically matched to the patients, thus avoiding the problem of rejection.

That could lead to developing stem cell therapies, although probably not for a decade or more.

"We are convinced that working with embryonic stem cells holds tremendous promise for the treatment of a host of currently intractable and incurable diseases," said Harvard University Provost Steven Hyman.

Hit or Miss Technology

Pursuing this promise is controversial. The field remains under a cloud cast by disgraced South Korean researcher Hwang Woo Suk, who a year ago published a fraudulent paper that claimed to have achieved what the Americans are now attempting.

Some conservative groups oppose the research -- and government rules block federal funding of it -- because it involves destroying human embryos.

What's more, the technology needed to tap the potential of genetically tailored stem cells is hit or miss, if not nonexistent.

The process starts with a patient's cell, such as a skin cell, and a woman's donated egg. The nuclear DNA from the patient's cell is inserted into the egg, which has been emptied of its own nucleus.

An electrical or chemical signal fuses this "cloned" organism, which begins dividing into an embryo that, at about 100 cells, briefly contains the wondrous stem cells that give rise to all specialized tissues and organs in the body.

Initial Focus on Diabetes

Only one team, at the University of Newcastle in the United Kingdom, has published credible evidence of cloning a human embryo to the 100-cell stage -- and it took 36 eggs donated by 11 women. The UK group did not try to harvest embryonic stem cells, much less coax them to differentiate into specialized cells.

Despite the obstacles, the American researchers said they are optimistic and determined.

Douglas Melton, co-director of Harvard's stem cell institute, and his colleague Kevin Eggan, a molecular biologist, have already spent more than two years going through scientific and ethical reviews to get approval for their work, which will be funded entirely with private and university money.

Federal funding is only available for research using about 20 embryonic stem cell lines created before federal restrictions took effect in 2002. Those colonies are considered old and difficult to use.

Initially, the pair will focus on diabetes -- a disease that afflicts Melton's son and daughter. Eggan also plans to study amyotrophic lateral sclerosis, or Lou Gehrig's Disease.

"Diabetes is a complex disease that involves many genes interacting with the environment," Melton said. "It's hard to get at the root cause. In essence, we want to use embryonic stem cells to move the study of the disease from the patient to the petri dish."

Cells Genetically Matched to Patients

George Daly, a Harvard Medical School professor and associate director of Boston Children's Hospital stem cell program, hopes to use patient-tailored stem cells to decipher and treat blood diseases such as sickle cell anemia and leukemia.

In 2002, Daly and colleagues used mice to successfully test the concept of therapeutic cloning. They cloned an embryo using an immune-deficient adult mouse cell, extracted stem cells from the embryo, repaired the genetic defect in some stem cells, then used those stem cells to restore immune function in the mutant mice.

"Since then, we have improved our methods considerably," Daly said.

At UCSF, researchers hope to create cell models of neurodegenerative diseases, including Parkinson's and Huntington's disease. Arnold Kriegstein, director of UCSF's Institute for Regeneration Medicine, said such models could be used to develop conventional drug therapies, as well as stem cell therapies.

By cloning embryos using nuclear DNA from sick people, the scientists aim to produce stem cells that are genetically matched to the patients, thus avoiding the problem of rejection.

Using human eggs, however, poses problems. Egg donors -- especially college women -- are paid up to $10,000 to undergo unpleasant hormone shots and the invasive extraction procedure.

To avoid the appearance of coercing donors -- a problem in the South Korean research -- the Harvard researchers will pay only for the women's medical expenses. Whether this will deter donations remains to be seen, Eggan said.

'Baby Cloning' Fear

Daly and the UCSF team initially will use eggs donated only after they fail to fertilize at IVF clinics. Such eggs are normally treated as waste.

"The availability of top quality eggs and embryos is extremely limited and this can slow the research," Daly said.

Opponents of the research were quick to condemn it.

"You can't do evil even if good comes out of it," said Judie Brown, president of the American Life League, an anti-abortion group.

David Christensen, director of Congressional affairs at the Family Research Council, raised the fear that successful human embryo cloning will "lead to baby cloning."

Stem cell experts say reproductive cloning -- an idea denounced by those on both sides of embryonic stem cell research -- is not possible with current technology, because scientists don't know how to artificially give the embryo certain genetic programming that is crucial to fetal development.

"The major power of [cloning] comes in the studying the development of human disease in human cells," said Johns Hopkins University bioethicist Debra Mathews. "It will be many years, at best, before patients benefit."