The gene that codes for the δ opioid receptor, opioid receptor delta 1 (OPRD1), is located on chromosome 1p34 [66]. Several studies have linked the association between variations in allelic frequencies of SNPs on the OPRD1 gene and opioid addiction. Mayer et al [76] studied a population of 103 German Caucasian individuals with heroin addiction and 115 control subjects, and reported the involvement of the C921T (rs2234918) SNP in exon 3 of the OPRD1 gene [76]. Compared to control subjects, both the frequency of the C-allele and C/C homozygote genotype were increased in heroin users (relative risk (RR) = 4.42, CI (95%) = 1.82-10.75) however the T to C transition did not directly alter the primary amino acid sequence, therefore it was unlikely that this polymorphism altered the functioning of the δ opioid receptor [76].
