For example, R. intestinalis may function in the modulation of the gut–brain axis by reducing the level of colonic 5-hydroxytryptamine (5-HT), inhibiting the expression of glial fibrillary acidic protein (GFAP), and alleviating the depression-like behaviors in colitis model mice (Xu et al., 2021). The gene discussed is GFAP; the disease is colitis.