Extremely unusual genetic conditions can reveal normal processes governing physiologic regulation and metabolism. Children with rare homozygous mutations in the leptin gene and complete leptin deficiency develop extreme hyperphagia and obesity soon after birth but respond with normal eating and a selective loss of excess body fat upon being given small amounts of leptin. Heterozygote relatives have 30% more fat than predicted and relatively low leptin levels. This demonstrates leptin's fundamental involvement in maintaining energy balance. Leptin also seems to act as a metabolic gate allowing children to enter puberty.