LEP and obesity due to melanocortin 4 receptor deficiency: However, when obesity is induced with the introduction of either leptin or leptin receptor knockout mutations (lep-ob and lep-db, respectively), both male and female DBA mice develop a diabetes-like phenotype due to β-cell failure, indicating the presence of diabetes risk genes in the DBA genome (Leiter, 1981; Chua et al., 2002).