Scientists are testing whether implanted stem cells can replace the dopamine-producing neurons destroyed by Parkinson’s disease. The hope is that restoring dopamine at its source could improve movement and slow the condition’s relentless progression. Credit: Shutterstock Parkinson's disease is a long term neurological condition that gradually worsens over time. More than one million people in the United States are living with the disorder, and about 90,000 new cases are diagnosed each year.