Bioengineered obese mouse, Aberdeen, Scotland, 1998. In the 1950s, scientists discovered a strain (a genetic variation) of unusually obese mice, but toiled for several decades to find out why they were so fat. Thirty years later a team led by Dr Jeffrey Friedman discovered that the mice were lacking a particular gene. In normal animals, this gene produced a protein called leptin that told the mouse when to stop eating. Without leptin it kept on eating, growing fatter and fatter. When Friedman gave the obese mice leptin they lost weight.