Parkinson's disease occurs when the neurons or nerve cells in the portion of the brain that controls movement stop functioning properly. these neurons send signals using a molecule called dopamine, and are referred to as dopaminergic neurons. about 500,000 people are currently living with parkinson’s disease in the u. s.. although there are drugs that can with symptoms of the disease there is no cure. treatments relieve the symptoms of parkinson's disease, but do not slow down or reverse the damage to nerve cells in the brain. over time, the clinical features get worse despite treatment. by the time patients are diagnosed with parkinson’s they have often had the disease for years and have lost over half of the dopamine cells within the brain. tests that detect parkinson’s earlier may , but scientists are searching for a way to replace the damaged cells. in recent years, the spotlight has been directed at this disease by celebrities such as michael j. fox much medical research and money has been directed at finding a cure for this disease. research has the potential to significantly impact the development of disease-modifying treatments for parkinson's disease, and considerable progress has been made in creating dopamine-producing cells. a) cloning b) stem cell c) human genome d) genetically modified cell