Single‐photon emission tomography (SPECT) was performed using iodine‐123‐β‐carbomethoxy‐3β‐(4‐iodophenyltropane) (I‐123 β CIT) as a tracer to measure in vivo DAT availability in 20 drug‐naive patients with ADHD [mean age ± standard deviation (SD)]: 25 ± 3.44 years; male:female = 11:9] and 20 age‐ and sex‐matched healthy controls (HCs) (mean age ± SD: 23.9 ± 2.27 years). This evidence concerns the gene SLC6A3 and attention deficit-hyperactivity disorder.