The ABCB1 gene is composed of 28 exons ranging in size from 49 to 209 base pairs, encoding an mRNA of 4.5 kb. The most common polymorphisms found in ABCB1 are 1236C>T, 2677 G>T/A/C, and 3435C>T. The transport protein P-glycoprotein, a product of the ABCB1 gene, plays an important role in the absorption and distribution of many drugs. P-glycoprotein is an ATPase-powered enzyme that transports substances, including opioids, across cell membranes and the blood–brain barrier [19,20], which regulates central nervous system exposure to drugs. It has been suggested that genetic variations in ABCB1 could be a cause of inter-individual differences in drug response [21]. In this study, we investigated the C3435T SNP in exon 26 of ABCB1, where C>T. Thus, the major allele is CC, the heterozygous minor allele is CT and the homozygous minor allele is TT.
