SLC6A3 and attention deficit-hyperactivity disorder: However, even if DAT deficiency can be considered an ideal model for investigating early Parkinson’s disease [39], DAT−/− mice exhibit phenotypic characteristics, such as dwarfism and growth deficit that make them unsuitable for the study of molecular mechanisms underlying gastrointestinal disorders, such as those associated to psychiatric pathologies (e.g., attention-deficit/hyperactivity disorder, schizophrenia or bipolar disorders).