The microbiome plays a vital role in the gut–brain axis underlying psychiatric disorders, including MDD.34,35 To investigate whether anxiety- and depression-like behaviors of Negr1 KO mice are associated with alterations in the gut microbiota, we compared fecal microbiota between Negr1 KO mice at the age of 9–11 wk (9 wk, n = 17) and age- and gender-matched WT mice (n = 12). This evidence concerns the gene NEGR1 and major depressive disorder.