Patients with major depressive disorder have been shown to have a surge in fecal Bacteroidetes, Proteobacteria, and Actinobacteria, as well as a decrease in fecal Faecalibacterium, all of which are associated with low levels of brain-derived neurotrophic factor (BDNF) in the serum [55]. The gene discussed is BDNF; the disease is major depressive disorder.