Numerous genetic studies have associated inheritance of APOE4 with increased risk, earlier onset, or poorer clinical outcome for a number of neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), traumatic brain injury, and HIV-encephalitis [2-10]. The gene discussed is APOE; the disease is Parkinson disease.