In this clinical article, Dr. K K Jain, a neurology consultant in Basel, Switzerland, describes gene therapy of various neurodegenerative disorders: Parkinson disease, Alzheimer disease, Huntington disease, and amyotrophic lateral sclerosis. Some of these are in clinical trials. Parkinson disease is an ideal candidate. The introduction of functional genes into the brain of patients with Parkinson disease may, for example, prove useful as a means to replace a defective gene, introduce a potentially neuroprotective or neurorestorative protein, or permit the physiological delivery of a deficient neurotransmitter. Potential gene therapy approaches to Alzheimer disease are based on neurotrophic factors and not neurotransmitters. The optimal method of gene therapy is ex vivo involving implantation of genetically engineered cells secreting nerve growth factor. A clinical trial for amyotrophic lateral sclerosis was conducted with implantation of genetically modified cells.