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Association between serotonin transporter gene polymorphisms and heroin dependence: a meta-analytic study

Authors Lin PY, Wu YS

Received 26 August 2016

Accepted for publication 16 November 2016

Published 30 November 2016 Volume 2016:12 Pages 3061—3067


Checked for plagiarism Yes

Review by Single anonymous peer review

Peer reviewer comments 2

Editor who approved publication: Professor Wai Kwong Tang

Pao-Yen Lin,* Yi-Shan Wu*

Department of Psychiatry, Kaohsiung Chang Gung Memorial Hospital and Chang Gung University College of Medicine, Kaohsiung, Taiwan

*These authors contributed equally to this work

Background: Studies have examined the association between heroin dependence and serotonin transporter gene polymorphisms but yielded inconsistent results. The purpose of current study is to determine the overall effect of these polymorphisms on the risk for heroin dependence through a meta-analytic method.
Methods: A meta-analysis was conducted to examine the association of heroin dependence with two common polymorphisms of serotonin transporter gene, in the promoter (5-hydroxytryptamine transporter-linked promotor region [5-httlpr]) and intron 2 (a various number tandem repeat in serotonin transporter intron 2 [STin2]). Data from studies with 5-httlpr (6 studies) and STin2 (8 studies) were synthesized by random effects model.
Results: In the analysis, heroin dependence was found to be significantly associated with the S allele of 5-httlpr (odds ratio [OR] =1.22, 95% confidence interval [CI] =1.08–1.41, P=0.002). The association between the S allele of 5-httlpr and heroin dependence was significant in Caucasian subjects (OR =1.37, 95% CI =1.12–1.68, P=0.003), but not in non-Caucasian subjects. On the other hand, no association with STin2 polymorphism was found (OR =1.14, 95% CI =0.91–1.42, P=0.242).
Conclusion: The results suggest an ethnic-specific effect of the 5-httlpr polymorphism on the risk for heroin dependence, but the influence of the genetic variance in the patients with comorbidities or intermediate phenotypes of heroin dependence needs to be further examined.

Keywords: ethnicity, heroin, polymorphism, serotonin transporter, STin2


Heroin dependence (HD) is associated with significant comorbidities (including viral hepatitis, human immunodeficiency virus infection, depression, and other substance abuse), serious socioeconomic consequences, and higher mortality rate.1 Although its pathogenesis is complex, the drug dependence has its origins in both genetic and environmental risk factors.2 However, the genetic factors contributing to the biological basis of HD have not yet been established. Previous literature has proposed that the serotonin (5-hydroxytryptamine, 5-HT) system, through modifying dopamine transmission, is involved in the deficiency in reward system in drug dependence.3 The study examining monoamine markers in the brains of chronic heroin users found decreased level of 5-hydroxyindoleacetic acid, a serotonin metabolite, in striatum.4 Neuroimaging studies have shown that the association between dopamine and serotonin transporter availability and addictive behaviors, indicating a role of serotonin in HD.57 Also, dysfunction of 5-HT system is associated with impulsivity, disordered behavioral inhibition, and depression, which are all frequently observed in drug addicts.8 Hence, in search of candidate genes for the susceptibility to HD, serotonergic pathway has received substantial attention in research.9

The 5-HT transporter (5-HTT), localized on the presynaptic membrane of serotonergic neurons, plays an important role in maintaining homeostasis of serotonin transmission by uptaking released serotonin from the synaptic cleft.10 The human 5-HTT protein is encoded by SLC6A4 gene, mapped to chromosome 17q11.1-q12.11 The gene expression is regulated by a functional polymorphism in the 5′ regulatory promotor region (termed 5-httlpr, 5-hydroxytryptamine gene-linked polymorphic region), involving two common alleles that correspond to a 44-base pair deletion (S allele) or insertion (L allele).12 The 5-httlpr S allele was found to reduce efficiency of transcription, resulting in decreased expression of 5-HTT and serotonin uptake in lymphoblast cell lines.13 The other common polymorphic variant in SLC6A4 gene, located in intron 2, consists of a variable number of 17-bp repeats (9, 10, or 12-repeat) (serotonin transporter intron 2 [STin2]). Meta-analytic studies have indicated that serotonin gene is associated with the susceptibility of a few psychiatric disorders, such as the risk of developing violent suicide,14 affective disorders,15 antidepressant response,16 and schizophrenia.17

In the last few years, many studies have reported that association between heroin abuse and serotonin gene,18,19 but the evidence remains inconclusive. Some of them showed the association with 10-repeat allele in STin220 or with SS genotype in 5-httlpr,21 but these positive findings were not replicated in other studies. The inconsistency may be due to insufficient statistical power in individual studies, poor phenotype definition, heterogeneity of subjects among different studies, or random error in the absence of a true association. To reconcile the discrepancy among studies, a meta-analysis was performed to determine the overall effect of these polymorphisms on the susceptibility to HD by synthesizing the results from all published articles.


Literature search

To identify studies eligible for this meta-analysis, a literature search was performed for all studies available up to April 2016 through PubMed at the National Library of Medicine using key words: (heroin OR opiate) AND (serotonin transporter OR SLC6A4 OR 5-httlpr OR STin2). References from identified original studies and relevant review articles were carefully screened to extract studies not indexed in the electronic database.

Inclusion criteria of studies

Included studies had to meet all the following criteria: 1) used a case-control design; 2) compared the STin2 or 5-httlpr of serotonin transporter gene between patients with HD and control subjects; 3) reported sufficient original data for analysis; and 4) were independent from other studies. One study that included and reanalyzed a previously published dataset was not regarded as an independent study; in this situation, only the study consisting of a larger sample size was included in the meta-analysis.

Meta-analytic methods

Separate meta-analyses were conducted for examination of the association of HD with 5-httlpr and with STin2. Hardy–Weinberg equilibrium was examined in all included studies. The strength of the association of HD with 5-httlpr and with STin2 was shown as the odds ratio (OR), whereby a greater value of OR indicates a direction of positive association of HD with 5-httlpr S allele, or with the 10-repeat allele in STin2. The results of individual studies were pooled by a random effects model,22 by which ORs were pooled and a 95% confidence interval (CI) of OR was computed. The significance of the pooled OR was determined by the z-test.

A homogeneity test (Q statistics) was conducted to assess whether the group of effect sizes (ln[OR]) came from a homogeneous source.23 A rejection of homogeneity suggests that there might be a systematic difference among included studies. If the pooled analysis showed significant association, a sensitivity test was performed to examine if any one study contributed significantly to the overall association. Publication bias was assessed by Egger’s linear regression analysis24 and funnel plots.

We performed meta-analysis by using Comprehensive Meta-Analysis, Version 2 (Biostat, Englewood, NJ, USA). Two-sided P-values <0.05 were considered statistically significant.


Through computerized database search, studies from 11 papers were initially identified.20,21,2533 Among them, 4 papers provided association data for both 5-httlpr and STin2 polymorphisms of 5-HTT gene.2730 The studies by Saiz et al29,30 shared the same research subjects, hence only the latter was included in the current meta-analysis. The percentages of alleles and genotypes of control subjects in all included studies were consistent with Hardy–Weinberg equilibrium.

First, the percentage of the S allele of 5-httlpr was compared between patients with HD and controls. Six studies, with 2,459 subjects (868 patients and 1,591 controls), were included into the analysis (Table 1). While only two studies showed a significant association of the S allele with HD, 21,32 other studies showed no association (Figure 1A). The homogeneity analysis showed a negative result (χ2=4.72, df=5, P=0.451), which suggested that the effect sizes from individual studies were not statistically different and justified the use of fixed effects model. The pooled OR from these studies was 1.23 (95% CI =1.08–1.41, P=0.002) (Figure 1A), indicating a small but significant association between HD and the S allele. In addition, sensitivity test found that the pooled OR remained significant after removing any one of the six included studies, indicating the significant association was not excessively influenced by any of them.

Table 1 Association studies of heroin dependence and 44-bp repeats polymorphism in serotonin transporter gene promoter
Abbreviation: DSM-IV, Diagnostic and Statistical Manual of Mental Disorders, 4th ed.

Figure 1 Odds ratios (ORs) and 95% confidence intervals (CIs) of individual studies and pooled data for all included association studies between heroin dependence and (A) the allelic distribution in serotonin transporter gene promoter polymorphism (5-httlpr), and (B) the allelic distribution in 17-bp variant number tandem repeats in intron 2 of 5-HTT gene (STin2).
Abbreviations: 5-httlpr, 5-hydroxytryptamine transporter-linked promotor region; STin2, serotonin transporter intron 2.

Although heterogeneity test of the included studies did not show significant difference in their effect sizes, ethnic difference may cause a differential distribution of genetic polymorphisms, evidenced by a significant increase in the L allele in the controls of three studies with Caucasian subjects21,28,29 than that in the others25,27,32 (56.8% vs 38.6%, P<0.001). So, the association of the 5-httlpr polymorphism with HD was also examined separately in studies with Caucasian and non-Caucasian subjects. Analyses of Caucasian subjects showed HD was significantly associated with the S allele (OR =1.37, 95% CI =1.12–1.68, P=0.003). However, analyses of non-Caucasian subjects showed no association (P=0.226). These results indicated ethnic heterogeneity in genetic susceptibility of 5-httlpr to HD.

Second, we compared allelic distribution of STin2 polymorphism of serotonin transporter gene between HD patients and controls. Eight studies, with 2,731 subjects (1,329 patients and 1,402 controls), were included into the analysis (Table 2). The paper by Galeeva et al contained data from two distinct ethnic groups (Russians and Tatars),26 which were regarded as two individual studies. Three different alleles, 9-repeat, 10-repeat, and 12-repeat, have been found among these studies. We omitted subjects carrying 9-repeat allele from our analysis because this allele was too rare (<1%) or there were none in individual studies. In our analysis, there was no association between HD and the 10-repeat allele (OR =1.14, 95% CI =0.91–1.42, P=0.242) (Figure 1B), nor was heterogeneity among the ORs of the studies (χ2=11.91, df=7, P=0.104). The result was still insignificant when limiting studies from either Caucasian or non-Caucasian subjects.

Table 2 Association studies of heroin dependence and 17-bp repeats polymorphism in intron 2 of serotonin transporter gene
Abbreviation: DSM-IV, Diagnostic and Statistical Manual of Mental Disorders, 4th ed.

Finally, by using linear regression analysis, there is significant publication bias for the association studies of HD with STin2 (P=0.043), but not with 5-httlpr (P=0.667). The results were also shown in the funnel plots (5-httlpr in Figure 2A, STin2 in Figure 2B).

Figure 2 Funnel plots examining publication bias in studies examining association between heroin dependence and (A) the allelic distribution in serotonin transporter gene promoter polymorphism (5-httlpr), and (B) the allelic distribution in 17-bp variant number tandem repeats in intron 2 of 5-HTT gene (STin2). The plots describe the relationship between effect sizes (log odds ratio) of studies and their precisions (inverse of standard error). The empty dots and diamond represent included studies and pooled effect size, respectively. The black dots represent imputed studies meant to balance skewed distribution of included studies in the funnel plot. The black diamond represents pooled effect size when considering imputed studies.
Abbreviations: 5-httlpr, 5-hydroxytryptamine transporter-linked promotor region; STin2, serotonin transporter intron 2.


Current analysis showed association of HD with the S allele of 5-httlpr polymorphism, whereas only two included reports showed such association (Figure 1).21,32 Expression of serotonin transporter protein was influenced by the 5-httlpr, where the L allele produced the expression level much higher than the S allele.12,13 Moreover, 5-HTT binding sites and mRNA levels in the dorsal raphe, median raphe, and the substantia nigra of postmortem brains also varied by 5-httlpr genotypes, with the highest levels from subjects with LL genotype rather than LS or SS genotype.34 Hence, our result suggested a role of lower activity of 5-HTT protein in the pathogenesis of HD. This result is supported by recent studies suggesting a linkage between HD and markers on the long arm of chromosome 17,35,36 where the serotonin transporter gene resides. Similar association between the low-expression allele of 5-httlpr and other addictive behaviors has been shown, such as alcohol dependence,37 pathological gambling,38 and excessive Internet use,39 although not with cocaine dependence.40

However, this effect was complicated by the later finding that an A/G single nucleotide polymorphism in the L allele of 5-httlpr contributed to the functional variation of the gene and was significantly associated with obsessive-compulsive disorder.41 Because L(G) and S alleles gave almost equivalent effect in gene expression, but lower than that of the L(A) allele. It provided functional triallelic variability that was previously unidentified. Subjects with LL or LS genotype in previous studies may carry the lower-expressing L(G) allele, thus may obscure a true effect of 5-httlpr on gene expression.

Although this study supports the role of the 5-httlpr polymorphism in the susceptibility to HD, caution needs to be taken to explain the results. HD is a highly heterogeneous disorder in its etiologies, clinical symptoms, and treatment response, which can be influenced by personality traits, or comorbid mental illnesses. Difference in these intermediate phenotypes may also result from genetic variability. Some studies have examined the genetic susceptibility to clinical characteristics and personality traits in heroin addicts. For example, Gerra et al found that SS genotype was significantly associated with violent offenders, but not with non-offenders with HD. Also, HD subjects with SS genotype showed higher levels of suspiciousness and negativism, compared to LL subjects.21 In addition, higher harm avoidance trait was found in HD patient groups, and was also associated with the interaction between DRD2, 5-httlpr, and ALDH2 genes.42 In the study of Galeeva et al, as a whole, no association between HD and STin2 polymorphism was found, but they found heterozygous 10-/12-repeats genotype was protective from early-onset opiate abuse.26 Moreover, a recent report showed a synergistic effect of a 5-HT2A polymorphism and both 5-httlpr and STin2 on the risk of HD.30 Whether the gene variant is associated with only a part of HD patients or with certain personality traits that predispose individuals to substance abuse needs further clarification.

In addition, population stratification is one important bias of association studies, especially in those carried out using population-based design. To overcome this possible bias, we stratified the studies by the primary ethnicity of the study sample and found significant association only in Caucasian samples. Such ethnicity-specific association was also observed in a recent meta-analysis of genetic studies of obsessive-compulsive disorder.43 Difference in allelic distribution in multicultural populations was supported in the association of the serotonin transporter gene with substance use disorder.44 However, addictive behaviors, such as HD, were shaped by environmental risk factors, which can be different among races. Family-based association studies can be performed to decrease the confounding effect related to population stratification.


In summary, although revealed significant association between HD and the S allele, the interpretation of the results was limited by small amount of included studies. Also, the existence of prevalent psychiatric comorbidities of in patients with HD may complicate the genetic effects on this disorder. These polymorphisms may play a more significant role in some endophenotypes or a subpopulation of the patients. Larger and more carefully designed studies are required to validate the role of 5-httlpr polymorphism in the susceptibility in HD.


This work received no research funding.

Author contributions

PY Lin designed the study, analyzed and interpreted data, wrote the initial draft, and finalized the manuscript. YS Wu wrote the initial draft and revised the manuscript. Both authors contributed toward data analysis, drafting and revising the paper and agree to be accountable for all aspects of the work.


The authors report no conflicts of interest in this work.



Degenhardt L, Bucello C, Mathers B, et al. Mortality among regular or dependent users of heroin and other opioids: a systematic review and meta-analysis of cohort studies. Addiction. 2011;106(1):32–51.


Kendler KS, Jacobson KC, Prescott CA, Neale MC. Specificity of genetic and environmental risk factors for use and abuse/dependence of cannabis, cocaine, hallucinogens, sedatives, stimulants, and opiates in male twins. Am J Psychiatry. 2003;160(4):687–695.


Comings DE, Blum K. Reward deficiency syndrome: genetic aspects of behavioral disorders. Prog Brain Res. 2000;126:325–341.


Kish SJ, Kalasinsky KS, Derkach P, et al. Striatal dopaminergic and serotonergic markers in human heroin users. Neuropsychopharmacology. 2001;24(5):561–567.


Cosgrove KP, Tellez-Jacques K, Pittman B, et al. Dopamine and serotonin transporter availability in chronic heroin users: a [(1)(2)(3)I]beta-CIT SPECT imaging study. Psychiatry Res. 2010;184(3):192–195.


Lin SH, Chen KC, Lee SY, et al. The association between availability of serotonin transporters and time to relapse in heroin users: a two-isotope SPECT small sample pilot study. Eur Neuropsychopharmacol. 2012;22(9):647–650.


Yeh TL, Chen KC, Lin SH, et al. Availability of dopamine and serotonin transporters in opioid-dependent users – a two-isotope SPECT study. Psychopharmacology (Berl). 2012;220(1):55–64.


Goodman A. Neurobiology of addiction. An integrative review. Biochem Pharmacol. 2008;75(1):266–322.


Kreek MJ, Bart G, Lilly C, LaForge KS, Nielsen DA. Pharmacogenetics and human molecular genetics of opiate and cocaine addictions and their treatments. Pharmacol Rev. 2005;57(1):1–26.


Bengel D, Murphy DL, Andrews AM, et al. Altered brain serotonin homeostasis and locomotor insensitivity to 3, 4-methylenedioxymethamphetamine (“Ecstasy”) in serotonin transporter-deficient mice. Mol Pharmacol. 1998;53(4):649–655.


Ramamoorthy S, Bauman AL, Moore KR, et al. Antidepressant- and cocaine-sensitive human serotonin transporter: molecular cloning, expression, and chromosomal localization. Proc Natl Acad Sci U S A. 1993;90(6):2542–2546.


Heils A, Teufel A, Petri S, et al. Allelic variation of human serotonin transporter gene expression. J Neurochem. 1996;66(6):2621–2624.


Lesch KP, Bengel D, Heils A, et al. Association of anxiety-related traits with a polymorphism in the serotonin transporter gene regulatory region. Science. 1996;274(5292):1527–1531.


Lin PY, Tsai G. Association between serotonin transporter gene promoter polymorphism and suicide: results of a meta-analysis. Biol Psychiatry. 2004;55(10):1023–1030.


Cho HJ, Meira-Lima I, Cordeiro Q, et al. Population-based and family-based studies on the serotonin transporter gene polymorphisms and bipolar disorder: a systematic review and meta-analysis. Mol Psychiatry. 2005;10(8):771–781.


Serretti A, Kato M, De Ronchi D, Kinoshita T. Meta-analysis of serotonin transporter gene promoter polymorphism (5-HTTLPR) association with selective serotonin reuptake inhibitor efficacy in depressed patients. Mol Psychiatry. 2007;12(3):247–257.


Fan JB, Sklar P. Meta-analysis reveals association between serotonin transporter gene STin2 VNTR polymorphism and schizophrenia. Mol Psychiatry. 2005;10(10):928–938, 891.


Cao J, LaRocque E, Li D. Associations of the 5-hydroxytryptamine (serotonin) receptor 1B gene (HTR1B) with alcohol, cocaine, and heroin abuse. Am J Med Genet B Neuropsychiatr Genet. 2013;162B(2):169–176.


Cao J, Liu X, Han S, Zhang CK, Liu Z, Li D. Association of the HTR2A gene with alcohol and heroin abuse. Hum Genet. 2014;133(3):357–365.


Tan EC, Yeo BK, Ho BK, Tay AH, Tan CH. Evidence for an association between heroin dependence and a VNTR polymorphism at the serotonin transporter locus. Mol Psychiatry. 1999;4(3):215–217.


Gerra G, Garofano L, Santoro G, et al. Association between low-activity serotonin transporter genotype and heroin dependence: behavioral and personality correlates. Am J Med Genet B Neuropsychiatr Genet. 2004;126(1):37–42.


Hedges LV. Fixed effects models. In: Cooper H, Hedges LV, eds. The Handbook of Research Synthesis. New York: Russell Sage foundation; 1994:285–299.


Shadish WR, Haddock CK. Combining estimates of effect size. In: Cooper H, Hedges LV, eds. The Handbook of Research Synthesis. New York: Russell Sage foundation; 1994:261–281.


Egger M, Davey Smith G, Schneider M, Minder C. Bias in meta-analysis detected by a simple, graphical test. BMJ. 1997;315(7109):629–634.


Kotler M, Cohen H, Kremer I, et al. No association between the serotonin transporter promoter region (5-HTTLPR) and the dopamine D3 receptor (BalI D3DR) polymorphisms and heroin addiction. Mol Psychiatry. 1999;4(4):313–314.


Galeeva AR, Gareeva AE, Iur’ev EB, Khusnutdinova EK. [VNTR polymorphisms of the serotonin transporter and dopamine transporter genes in male opiate addicts]. Mol Biol (Mosk). 2002;36(4):593–598.


Li T, Liu X, Zhao J, et al. Allelic association analysis of the dopamine D2, D3, 5-HT2A, and GABA(A)gamma2 receptors and serotonin transporter genes with heroin abuse in Chinese subjects. Am J Med Genet. 2002;114(3):329–335.


Szilagyi A, Boor K, Szekely A, et al. Combined effect of promoter polymorphisms in the dopamine D4 receptor and the serotonin transporter genes in heroin dependence. Neuropsychopharmacol Hung. 2005;7(1):28–33.


Saiz PA, Garcia-Portilla MP, Florez G, et al. Differential role of serotonergic polymorphisms in alcohol and heroin dependence. Prog Neuropsychopharmacol Biol Psychiatry. 2009;33(4):695–700.


Saiz PA, Garcia-Portilla MP, Arango C, et al. Association between heroin dependence and 5-HT2A receptor gene polymorphisms. Eur Addict Res. 2008;14(1):47–52.


Yang M, Kavi V, Wang W, Wu Z, Hao W. The association of 5-HTR2A-1438A/G, COMTVal158Met, MAOA-LPR, DATVNTR and 5-HTTVNTR gene polymorphisms and antisocial personality disorder in male heroin-dependent Chinese subjects. Prog Neuropsychopharmacol Biol Psychiatry. 2012;36(2):282–289.


Wang TY, Lee SY, Chung YL, et al. TPH1 and 5-HTTLPR genes specifically interact in opiate dependence but not in alcohol dependence. Eur Addict Res. 2016;22(4):201–209.


Yang M, Mamy J, Wang Q, et al. The association of 5-HTR2A-1438A/G, COMTVal158Met, MAOA-LPR, DATVNTR and 5-HTTVNTR gene polymorphisms and borderline personality disorder in female heroin-dependent Chinese subjects. Prog Neuropsychopharmacol Biol Psychiatry. 2014;50:74–82.


Little KY, McLaughlin DP, Zhang L, et al. Cocaine, ethanol, and genotype effects on human midbrain serotonin transporter binding sites and mRNA levels. Am J Psychiatry. 1998;155(2):207–213.


Glatt SJ, Su JA, Zhu SC, et al. Genome-wide linkage analysis of heroin dependence in Han Chinese: results from wave one of a multi-stage study. Am J Med Genet B Neuropsychiatr Genet. 2006;141B(6):648–652.


Gelernter J, Panhuysen C, Wilcox M, et al. Genomewide linkage scan for opioid dependence and related traits. Am J Hum Genet. 2006;78(5):759–769.


McHugh RK, Hofmann SG, Asnaani A, Sawyer AT, Otto MW. The serotonin transporter gene and risk for alcohol dependence: a meta-analytic review. Drug Alcohol Depend. 2010;108(1–2):1–6.


Perez de Castro I, Ibanez A, Saiz-Ruiz J, Fernandez-Piqueras J. Genetic contribution to pathological gambling: possible association between a functional DNA polymorphism at the serotonin transporter gene (5-HTT) and affected men. Pharmacogenetics. 1999;9(3):397–400.


Lee YS, Han DH, Yang KC, et al. Depression like characteristics of 5HTTLPR polymorphism and temperament in excessive internet users. J Affect Disord. 2008;109(1–2):165–169.


Patkar AA, Berrettini WH, Hoehe M, et al. Serotonin transporter (5-HTT) gene polymorphisms and susceptibility to cocaine dependence among African-American individuals. Addict Biol. 2001;6(4):337–345.


Hu XZ, Lipsky RH, Zhu G, et al. Serotonin transporter promoter gain-of-function genotypes are linked to obsessive-compulsive disorder. Am J Hum Genet. 2006;78(5):815–826.


Wang TY, Lee SY, Chen SL, et al. Association between DRD2, 5-HTTLPR, and ALDH2 genes and specific personality traits in alcohol- and opiate-dependent patients. Behav Brain Res. 2013;250:285–292.


Bloch MH, Landeros-Weisenberger A, Sen S, et al. Association of the serotonin transporter polymorphism and obsessive-compulsive disorder: systematic review. Am J Med Genet B Neuropsychiatr Genet. 2008;147B(6):850–858.


Cao J, Hudziak JJ, Li D. Multi-cultural association of the serotonin transporter gene (SLC6A4) with substance use disorder. Neuropsychopharmacology. 2013;38(9):1737–1747.

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