More than a millon Americans who suffer from the debilitating neurological disorder Parkinson’s disease are likely to be among the first to benefit from promising advances in embryonic stem cell research, unless political controversy keeps slowing down the process, scientists said Monday.

Researchers said that while stems cells have shown promise as a treatment for diseases that involve degeneration of brain cells, such as Alzheimer’s and Lou Gehrig’s disease, the most promising research has involved Parkinson’s.

“We want to replenish the population of nerve cells that are dying in the brains of Parkinson’s victims,” Dr. Ole Isacson, a professor of neurology and neuroscience at the Harvard Medical School, said in an interview. “The encouraging recent scientific finding is that human embryo [embryonic] stem cells can be grown into the specific dopamine neuron that dies in Parkinson’s, and we know that it can be transplanted and restore function in animal models [rats].”

Parkinson’s is caused by the degeneration of neurons, or brain cells, that control movement. That results in a shortage of the brain-signaling chemical dopamine, which is why, Isacson said, scientists are working now to grow new dopamine-secreting neurons from embryonic and adult stem cells.

Embryonic stem cells are taken from early embryos, mostly obtained from fertilized eggs left over from in vitro fertilization clinics. They have captivated scientists with their ability to develop into any tissue in the human body, including all of 300 specific nerve cell types that make up the brain.

Adult stem cells are less flexible and can’t transform into as wide a range of alternate tissue.

Though U.S. researchers are struggling with the president’s limitations on their field, experiments are going full force in Canada and Europe. In the United States, development of dopamine cell replacement therapy will depend on finding and producing alternative sources of cells for transplants, Isacson said.

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