What use is a fat mouse? Mice of all colours, shapes and sizes were bred for fun in China and Japan in the 1600s. Victorian Britain also had a ‘mouse craze’, with the foundation of the National Mouse Club – by humans – in 1895. Breeders competed for prizes for new varieties of fancy mice with names like ‘red cream’, ‘ruby-eyed yellow’ and ‘creamy buff’. But what has this got to do with science? The new varieties were highly desirable, and mouse fanciers were creating masses of genetically identical mice. Since the early 1900s scientists have used some of these mice to investigate heredity and disease. 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.