Scientists could also use cloned stem cell lines to create new diagnostic tests or tools for drug discovery. "We have nothing in the genetic tool box to diagnose what's happening early on in a disease," says Renee A. Reijo Pera, codirector of the human stem cell biology program at the University of California, San Francisco. "Embryonic stem cells let you go back to day-one of the disease."

In Alzheimer's disease, for example, the brain is significantly damaged by the time the patient sees a doctor. To search for early signs of the disorder, scientists could generate stem cells using DNA from an Alzheimer's patient, then coax those cells to differentiate into neurons, monitoring the cells for specific proteins or other molecular changes that are different from neurons derived from healthy embryonic stem cells. The same approach might work with cancer, which is characterized by a series of harmful genetic changes. "We want to know what's the earliest you can detect differences in disease cells," says Reijo Pera, who is planning a new research program in human therapeutic cloning.

Scientists also hope that cloning stem cells will bring the field one step closer to a major goal of stem cell biology: to take an adult cell and turn it back into a stem cell without the use of human eggs. This feat would ultimately circumvent the biggest ethical concerns surrounding therapeutic cloning -- the creation and destruction of a human embryo. Scientists hope that if they can understand how an egg "reprograms" an adult cell into its undifferentiated state, they can eventually eliminate the need for human eggs and embryos.