The negative correlation is consistent with previous studies of depressive symptoms in Parkinson disease[47,48]; however, the relationship could not be found in patients with MD only[49,50] or patients with AD only.[51] Although prior studies analyzed the relationship between depressive symptoms and DAT availability in the brain, these studies used different patient groups, which might have affected the measures of dopamine availability in the brains of patients with depression. This evidence concerns the gene SLC6A3 and depressive symptom measurement.