BDNF is involved in manifold systemic metabolic processes of the body, and therefore a relatively high concentration of 32.69 ± 8.33 ng/mL BDNF is present in human blood serum [28] and down-regulated during several diseases, e.g., systemic lupus erythematosus [29] and schizophrenia [30]. The gene discussed is BDNF; the disease is schizophrenia.