DRD4 and bipolar disorder: Importantly, in addition to this association with FASD, genetic and epigenetic variation in Drd4 has been linked to attention deficit hyperactivity disorder (ADHD), schizophrenia, bipolar disorder, substance-use disorders, and several other neurobiological disorders (Bau et al., 2001; Chen et al., 2011; Ptácek et al., 2011; Docherty et al., 2012; Kordi-Tamandani et al., 2013; Zhang et al., 2013; Cheng et al., 2014; Faraone et al., 2014; Dadds et al., 2016; Ji et al., 2016).