A Pubmed search through the English Literature was conducted from 1979 to 2010 using various combinations of the following key words: “adiponectin”, “amylin”, “bariatric surgery”, “gastric bypass”, “gastrointestinal hormones”, “GLP-1”, “ghrelin”, “gut hormones”, “insulin”, “leptin”, “metabolic surgery”, “obesity”, “oxyntomodulin”, “peptide YY” (PYY), and “Roux-en-Y gastric bypass” (RYGB). The gene discussed is PYY; the disease is obesity due to melanocortin 4 receptor deficiency.