Circulating levels are inversely correlated to obesity and BMI, waist circumference, and leptin in healthy subjects; the normal level is reported in 0.37 μg/mL, but are significantly reduced in Crohn's disease, synovial fluid of rheumatoid arthritis patients, other inflammatory diseases, and obese individuals to 0.31 μg/mL [116]. The gene discussed is LEP; the disease is obesity due to melanocortin 4 receptor deficiency.