Scientists often use embryonic and induced pluripotent stem cells or EPS and iPS cells in their research. These stem cells are used in disease modeling and drug screening because they can divide indefinitely without differentiating, and, when stimulated, they can differentiate into most body cell types. Disease modeling is when cells are used to simulate a specific disease. In Parkison's disease or PD, neurons of the midbrain are damaged or lost and may not be suitable for disease modeling. Therefore, fibroblasts from a PD patient can be isolated and reprogrammed to form iPS cells. These cells can differentiate into specific midbrain neurons, which have been used to identify mutations that cause PD and to understand other disease mechanisms. Scientists also use iPSCs as alternatives to animal models for screening drugs. Patient-derived iPSCs are genetically similar to the patient's own cells. When these iPSCs differentiate, the cells can be used to test specific drugs for toxicity, efficiency, and the patient's response to the drug.