DRD4 and attention deficit-hyperactivity disorder: Candidate gene studies on ADHD’s DNA methylation profile based on peripheral samples such as blood or saliva have shown different methylation patterns of genes involved not only in dopaminergic, serotonergic, and neurotrophic systems including SLC6A4, DRD4, COMT, BDNF, and NGFR, but also neurotransmitter release or neurite outgrowth including ERC2 and CREB5 and associated with the symptoms and severity of ADHD (van Mil et al., 2014; Park et al., 2015; Xu et al., 2015; Dadds et al., 2016; Heinrich et al., 2017; Sengupta et al., 2017; Neumann et al., 2020).