GRM5 and major depressive disorder: mGluR5 is expressed at high levels in several brain regions (Brain tissue expression of GRM5, n.d.) and is involved in a multitude of brain-related illnesses (Su et al., 2022) including fragile X syndrome (FXS) (Bear et al., 2004; Brašić et al., 2022), depression (Pilc et al., 2008; Esterlis et al., 2022), Parkinson’s disease (Pisani et al., 2003; Azam et al., 2022), Alzheimer’s disease (Sokol et al., 2011), attention deficit hyperactivity disorder (ADHD) (Elia et al., 2010), and addictions (Niedzielska-Andres et al., 2021).