OXT and obesity due to melanocortin 4 receptor deficiency: Haploinsufficiency of single-minded 1 gene (SIM1), a transcription factor involved in the development of the PVN and SON of the hypothalamus, results in reduced expression of oxytocin in the PVN in mice and is associated with hyperphagia, decreased energy expenditure, and obesity that improve with oxytocin administration [130,131].