It is worth noting that numerous animal model studies have extensively revealed potential sex-specific mechanisms of MDD (Eid et al., 2019; Hao et al., 2019; Ma et al., 2019; Palanza, 2001), including serotonin, the hypothalamic–pituitary–adrenal axis, brain-derived neurotrophic factor, etc. Therefore, we can further explore how these sex-specific mechanisms affect the whole development of MDD by comparing the similarities and differences between animal model experiments and human MDD experiments. The gene discussed is BDNF; the disease is major depressive disorder.