The ob/ob or obese mouse is a mutant mouse that eats excessively and becomes profoundly obese. It is an animal model of type II diabetes. Identification of the gene mutated in ob led to the discovery of the hormone leptin, which is important in the control of appetite. The first ob/ob mouse arose by chance in a colony at the Jackson Laboratory in 1949. The mutation is recessive. Mutant mice are phenotypically indistinguishable from their unaffected littermates at birth, but gain weight rapidly throughout their lives, reaching a weight three times that of unaffected mice. ob/ob mice dev...