The synthesis of BDNF in neurons, which is significantly diminished in MDD, is closely associated with elevated methylation levels in the BDNF promoter region, and given that the methylation status of the BDNF gene has been implicated as a crucial factor in the pathogenesis of MDD, it is increasingly recognized as a potential biomarker for the disorder. The gene discussed is BDNF; the disease is major depressive disorder.