Several studies have shown that the expression of the MMP-9 protein decreased in the hippocampus of FTO knockout mice, and MMP-9 participated in the regulation of anxiety- and depression-like behaviors in mice by affecting the transformation of brain-derived neurotrophic factor precursor (proBDNF) to mature brain-derived neurotrophic factor (mBDNF). This evidence concerns the gene BDNF and depressive symptom measurement.