Deficiency in either leptin or its receptor, Lepr, leads to obesity due to increased feeding (Alingh Prins et al., 1986, McLaughlin and Baile, 1981) and reduced energy expenditure (through a decrease in core body temperature) (Trayhurn, 1979, Trayhurn et al., 1977). Here, LEP is linked to obesity due to melanocortin 4 receptor deficiency.