Researchers say they found that adult blood stem cells don't produce other types of tissue, a finding that raises new doubts about their use in treating such conditions as Parkinson's disease.
The study from Stanford University scientists supports the view that embryonic stem cells offer the most promise for treating conditions such as heart disease and spinal injury.
Many people, including President Bush, oppose using human embryonic stem cells in medical research because it involves the destruction of the embryo.
The research appears in Science magazine. It's part of a new field of study called regenerative medicine, which would cure or control disease by replacing flawed cells with new ones grown from stem cells.