DRD4 and heroin dependence: The –521 C/T polymorphism in the promoter region of the DRD4 gene influences its transcription level (decreased by 40% in T/T homozygotes) [40], and the presence of the C allele was previously shown to be associated with schizophrenia development risk [38, 40], whereas the T allele was found to predispose to heroin addiction [41].