In search for an explanation for these findings, a single photon emission computed tomography (SPECT) study in 24 ADHD patients with (n = 8) and without (past) cocaine dependence (n = 16) showed a lower striatal dopamine transporter (DAT) occupancy by MPH in cocaine dependent compared with non-dependent ADHD patients after two weeks of MPH treatment. Here, SLC6A3 is linked to attention deficit-hyperactivity disorder.