Among others, genes coding for proteins involved in 5-HT system such as the 5-HT transporter (SERT or 5-HT-T) [17-21], carriers for neutral amino acids (including tryptophan) [22] and 5-HT receptor subtypes [23-28] appear to be functionally relevant in either animal or human obesity. The gene discussed is SLC6A4; the disease is obesity due to melanocortin 4 receptor deficiency.