SLC6A3 and attention deficit-hyperactivity disorder: Patients with ADHD, in comparison to healthy controls, have higher dopamine transporter density and increased dopamine transporter binding, resulting in low levels of dopamine (Sulzer et al., 2005) Critically, in neurotypical adults, mesolimbic dopamine capacity assessed using positron emission tomography (PET), has been linked to functional integrity of the SN (McCutcheon et al., 2019).