LEP and obesity due to melanocortin 4 receptor deficiency: In addition, it has been shown in humans that serum leptin concentrations are positively correlated with obesity-related traits.34 After the identification of the leptin gene (lep) in mice, leptin mutations were also found in human patients, including a homozygous reading frame shift, a series of missense mutations and a number of polymorphisms that were associated with obesity.35–37