Human induced pluripotent stem (iPS) cells and regenerative medicine. Patients with Parkinson's disease have substantially reduced dopaminergic (tyrosine hydroxylase-positive, or TH+) neurons in their substantia nigra. Human iPS cells can be made by reprogramming adult somatic cells (that is, fibroblasts) from patients and healthy donors. iPS cells can be differentiated into neurons to investigate disease progression in vitro. Once their genetic defects are corrected, iPS cells of patients may be differentiated into functional neurons. High-purity dopaminergic neurons may be generated and transplanted as a treatment for patients with Parkinson's disease.