DRD4 and obesity due to melanocortin 4 receptor deficiency: Despite the limited expression in the brain compared to the other receptor subtypes, the DRD4 is particularly interesting because of the highly polymorphic nature of the human gene encoding for this protein [16,17] and because several human studies revealed DRD4 possible involvement in neuropsychiatric diseases, including attention deficit hyperactivity disorder (ADHD) [18,19], personality traits of novelty seeking [20,21], schizophrenia [22,23], psychostimulants addiction [24], mood disorders [25], eating disorders (ED) [26,27,28] and obesity [29,30].