Obesity and diabetes mellitus are associated with low or elevated serum leptin and insulin levels (U-like relation). Mutations in LEP and INS are linked to leptin and insulin decrease while mutations in LEPR and INSR to their increase. Homozygous LEP mutations are associated with the early onset of severe obesity and diverse impairment of physiological functions. Recessive LEPR mutations are associated with similar pathology in homozygous state. Missense mutations of INS are dominant. They induce synthesis of chimeric pro-insulin that may interfere folding and processing of active insulin molecules. In the heterozygous state they cause insulin deficiency and PND. Recessive INS mutations do not induce synthesis of anomalous pro-insulin, and they are associated with PND only in homozygous state. Mutations of INSR induce insulin resistance, lipodystrophy, other pathology, and suggest an important role of insulin in glucose level regulation and in stimulation of fat accumulation as well.