Inclusion criteria for patients with essential tremor were: (i) integrity of the nigrostriatal dopaminergic terminals, as evidenced by a normal dopamine transporter scan, to exclude parkinsonisms; (ii) no traumatic brain injury and past or current substance abuse, particularly alcohol; (iii) no dementia according to the DSM-IV; in particular, probable Alzheimer's disease was excluded according to the NINCDS-ADRDA criteria. This evidence concerns the gene SLC6A3 and Parkinson disease.