A protein comprising 167 amino acids that is important in the regulation of body weight, metabolism, and reproduction. Leptin was discovered in 1994, when it was identified as the missing protein in mice with a spontaneous single-gene defect that caused obesity. These ob/ob mice were very obese, had many of the metabolic abnormalities associated with obesity, and were infertile. However, when leptin levels were examined in other animal models of obesity and in obese humans, the levels were found to be elevated and not low or absent (as in the ob/ob mice). Thus, general obesity (not due to leptin deficiency) has been termed a leptin-resistant state. Since these initial findings, the biology of leptin has proven to be more complex than originally thought.