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 decreases in leptin and insulin while mutations in LEPR and INSR are linked to their increase. Homozygous LEP mutations are associated with the early onset of severe obesity and the diverse impairment of physiological functions. The recessive LEPR mutations are associated with similar pathology in homozygous state. Missense mutations of INS are dominant and induce the synthesis of chimeric proinsulin, which may interfere with the folding and processing of active insulin molecules. In the heterozygous state, they cause insulin deficiency and PND. Recessive INS mutations do not induce the synthesis of anomalous proinsulin, and they are only associated with PND in the homozygous state. Mutations of INSR induce insulin resistance, lipodystrophy, other pathologies, and suggest the important role of insulin in glucose level regulation and in the stimulation of fat accumulation.