Although only a few animal studies have investigated the relationship between the hypocretin system and MDD, there have some clinical investigations have demonstrated elevated levels of plasma hypocretin-1 in MDD patients, whereas others have found no significant association between plasma hypocretin-1 levels and the presence of depression (Tsuchimine et al., 2019; Akça et al., 2021; Li et al., 2021; Chen et al., 2024). The gene discussed is HCRT; the disease is major depressive disorder.