IL-34 is detectable at low concentrations in serum/plasma, cerebral spinal fluid, synovial fluid, and saliva [35, 55, 56] and there exists a correlation between levels of IL-34 secreted into these extracellular biofluids and disease parameters in rheumatoid arthritis, heart failure, viral infections, sepsis, periodontal disease, non-alcoholic fatty liver disease, obesity, and type 2 diabetes mellitus [35, 55–57]. The gene discussed is IL34; the disease is obesity due to melanocortin 4 receptor deficiency.