Meta-analyses and genome-wide association studies (GWAS), however, have found associations between Grm4 and several psychiatric disorders that share the impulsivity domain (see Table 1), such as PD, bipolar disorder, and ADHD (Carvalho-Silva et al., 2019; Fliers et al., 2012; Pletscher-Frankild et al., 2015). Here, GRM4 is linked to attention deficit-hyperactivity disorder.