DRD4 and schizophrenia: The DA D4 receptor gene (DRD4) is also a candidate gene for schizophrenia (Shi et al., 2008) and the −521 single nucleotide polymorphism (SNP) refers to a C-to-T substitution in the DRD4 promoter region (rs1800955) with the T allele resulting in 40% less transcriptional efficiency (Okuyama et al., 1999).