SLC6A3 and attention deficit-hyperactivity disorder: A growing amount of evidence suggests that the decrease of DAT levels, as well as the polymorphisms of its gene (Slc6a3), are etiopathogenetic factors for the development of a wide number of DA-related disorders, including Parkinson’s disease (PD) [8,9], attention deficit hyperactivity disorder (ADHD) [9], post-traumatic stress disorder [10], drug abuse, obsessive-compulsive disorder (OCD) [11], and bipolar depression (BD) [12].