Quercetin as Zinc Ionophore and Antiviral

Zinc has been recognized as broadly antiviral for decades, with intracellular zinc shown to inhibit the RNA-dependent RNA polymerase of many viruses including rhinoviruses, influenza, and a range of picornaviruses. The catch is that the divalent zinc cation Zn(II) does not cross lipid bilayers efficiently on its own — intracellular zinc concentrations are tightly controlled by membrane transporters (the ZIP and ZnT families) and elevating intracellular zinc enough to reach antiviral concentrations is biologically difficult. In 2014, Dabbagh-Bazarbachi and colleagues at the University of Lleida demonstrated in a clean liposome and cell-culture system that quercetin acts as a zinc ionophore — it binds zinc, the resulting lipophilic complex passes through cell membranes, and intracellular zinc rises measurably. This mechanism, combined with quercetin's direct activity against viral proteases, polymerases, and entry receptors documented across rhinovirus, influenza A, Epstein-Barr virus, herpes simplex, and hepatitis C in-vitro systems, is the foundation of the modern quercetin + zinc + vitamin C protocols used for upper-respiratory infection prophylaxis and early treatment. This page walks the mechanism, the virus-by-virus in-vitro evidence, the dosing rationale behind the combination, and the formulation choices that determine whether oral quercetin actually reaches the tissues where it needs to work.

Table of Contents

  1. Zinc as a Broad-Spectrum Antiviral — The Polymerase Connection
  2. The Ionophore Mechanism — Dabbagh-Bazarbachi 2014
  3. Rhinovirus — The Common Cold Target
  4. Influenza A — Wu 2016 and the Hemagglutinin Block
  5. Epstein-Barr Virus — Reactivation Suppression
  6. Herpes Simplex Virus (HSV-1 and HSV-2) In Vitro
  7. Hepatitis C — NS3 Protease and Heat-Shock Inhibition
  8. Dosing — The Quercetin + Zinc + Vitamin C Stack
  9. Bioavailability — Phytosome vs Raw Aglycone
  10. Cautions and Drug Interactions
  11. Key Research Papers
  12. Connections

Zinc as a Broad-Spectrum Antiviral — The Polymerase Connection

The antiviral pedigree of zinc predates almost every modern antiviral drug. Lozenge-form zinc was shown to shorten common-cold duration in randomized trials beginning in the 1980s, with the Hemilä 2017 meta-analysis pooling 575 lozenge-trial participants and confirming an approximately one-third reduction in cold duration when high-dose zinc acetate or zinc gluconate lozenges (75 mg or more daily of elemental zinc, dissolved slowly in the mouth) were started within 24 hours of symptom onset. The lozenge route is not coincidental — it delivers zinc directly to the upper-respiratory epithelium where rhinovirus replicates.

The molecular basis of the antiviral effect was clarified by te Velthuis and colleagues at Leiden University in 2010. Working in a cell-culture system with arterivirus and SARS-coronavirus (a non-pandemic veterinary and laboratory model), they showed that intracellular zinc directly inhibits the viral RNA-dependent RNA polymerase (RdRp) at concentrations that are achievable inside cells if zinc is delivered with an ionophore. The polymerase effect is broad — the same general mechanism applies to picornaviruses (including all of the rhinoviruses), orthomyxoviruses (influenza), flaviviruses (hepatitis C), and several other RNA-virus families. Zinc also stabilizes the secondary structure of certain viral RNAs and disrupts the function of viral zinc-finger proteins that depend on tightly coordinated zinc binding to maintain their conformations.

The pharmacological problem is selective delivery. Plasma zinc concentrations are tightly buffered at roughly 12-15 µmol/L by zinc-binding proteins (primarily albumin), and increasing dietary zinc above the homeostatic ceiling does not meaningfully raise free plasma zinc and does not raise intracellular zinc in most tissues. The cell-surface zinc transporters (ZIP1-14 importers, ZnT1-10 exporters) maintain a steep concentration gradient with intracellular free zinc held at picomolar levels — orders of magnitude below the concentrations needed to inhibit viral polymerases. To raise intracellular zinc enough to be antiviral, a molecule has to escort zinc across the lipid bilayer in defiance of the transporter system. That escort molecule is what is meant by a zinc ionophore.

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The Ionophore Mechanism — Dabbagh-Bazarbachi 2014

The classical ionophores of pharmacology are drug molecules with lipophilic exteriors and polar interior cavities that bind metal cations — ionomycin and A23187 for calcium, monensin and nigericin for sodium and potassium, valinomycin for potassium. For zinc, the academic-pharmacology ionophore is the anti-amebic drug clioquinol (and the related metal-chaperone compound PBT2), which were developed for Alzheimer's and prion-disease research. Clioquinol and pyrithione are potent zinc ionophores but are pharmaceutical agents with notable toxicity at therapeutic concentrations and are not used as dietary supplements.

The 2014 paper from Hamed Dabbagh-Bazarbachi, María-José Motílva, and Juan B. Fernandez-Larrea at the Universitat de Lleida in Spain (published in the Journal of Agricultural and Food Chemistry) tested whether dietary polyphenols had measurable ionophore activity. They constructed a liposome system using the zinc-sensitive fluorophore FluoZin-3 trapped inside the liposome aqueous compartment, with zinc and the candidate polyphenol added to the external buffer. Any compound that escorted zinc across the lipid bilayer would produce a FluoZin-3 fluorescence increase inside the liposome. They then cross-validated the most active hits in HepG2 hepatoma cells using FluoZin-3 AM as the intracellular zinc probe.

Quercetin emerged as a robust zinc ionophore in both systems. The catechol B-ring (3',4'-dihydroxyphenyl) and the 3-hydroxy-4-keto group on the C-ring of quercetin together form a bidentate chelation site that binds Zn(II). The resulting quercetin-zinc complex is more lipophilic than free zinc (which is highly hydrated and polar) and partitions efficiently into the lipid bilayer. Once inside the cell, the lower pH of the cytoplasm and competition from intracellular zinc-binding proteins (metallothionein, zinc-finger proteins) release the zinc, leaving it free to do whatever zinc does intracellularly — including inhibiting viral polymerases. The paper also identified epigallocatechin gallate (EGCG, the major green-tea catechin) as a zinc ionophore, with a somewhat different chelation geometry but similar functional effect.

The Dabbagh-Bazarbachi finding is in-vitro and on isolated liposome systems and cultured hepatoma cells; it has not been directly demonstrated in human tissue with supplement-dose oral quercetin. The plasma levels of total quercetin metabolites after standard oral dosing (1-2 µmol/L peak) are at the low end of the in-vitro effective range, and the metabolite forms (glucuronides and sulfates) are poorer ionophores than the free aglycone. The ionophore hypothesis is mechanistically clean and biochemically rigorous, but the in-vivo translation to clinical antiviral effect rests on the additional in-vitro and observational data summarized below rather than on direct human ionophore pharmacology trials.

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Rhinovirus — The Common Cold Target

Human rhinovirus (HRV) is a positive-sense single-stranded RNA picornavirus and the most common cause of the upper-respiratory illness colloquially called the common cold. More than 160 distinct serotypes have been identified, divided into three species (HRV-A, HRV-B, and the more recently characterized HRV-C). Most serotypes use ICAM-1 as the cellular receptor (the “major group” rhinoviruses); a smaller subset uses LDL-receptor family members (the “minor group” rhinoviruses). Rhinovirus replicates preferentially at the slightly cooler 33-35°C temperature of the upper-respiratory mucosa, which is the simple anatomic reason it produces upper-respiratory rather than lower-respiratory illness.

Ganesan and colleagues at the University of Virginia reported in 2012 (published in Virology) that quercetin suppressed rhinovirus replication in primary human bronchial epithelial cells through multiple mechanisms: reduced viral attachment, reduced viral endocytosis, reduced expression of the rhinovirus receptor ICAM-1 on the epithelial surface, and downstream inhibition of NF-kappaB-driven inflammatory cytokine secretion (IL-6, IL-8, CXCL10). The effect was concentration-dependent in the 1-50 µM range, with measurable suppression at the lower end of that range and near-complete suppression at 50 µM. A 2014 follow-up in mice infected with mouse-adapted HRV1B showed that oral quercetin pre-treatment reduced viral titers in lung tissue and reduced airway inflammation by approximately 50%.

The clinical translation in humans is suggestive but not definitive. The Heinz 2010 trial randomized 1,002 community-dwelling adults to placebo, quercetin 500 mg/day, or quercetin 1000 mg/day during the 12-week winter respiratory-illness season. The primary outcome (overall upper-respiratory-tract infection incidence and severity) was not significantly improved across the whole cohort, but a pre-specified subgroup analysis in fit and middle-aged-and-older participants (45 years and older with VO2max above the 25th percentile) showed a statistically significant reduction in upper-respiratory illness severity at the 1000 mg/day dose. The mixed result reflects the difficulty of running a single trial across a broad heterogeneous population — the subset most likely to benefit (older adults with reasonable baseline fitness) showed the expected protective effect.

The practical regimen for cold prophylaxis combines daily quercetin with daily zinc and vitamin C, especially during the autumn-winter peak rhinovirus season. The combination is not a guarantee of avoiding any cold, but the cumulative evidence supports a modest reduction in incidence and a more substantial reduction in severity and duration when an infection does establish.

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Influenza A — Wu 2016 and the Hemagglutinin Block

Influenza A is a negative-sense segmented single-stranded RNA orthomyxovirus that produces seasonal epidemics with substantial morbidity and mortality, particularly in older adults and patients with cardiopulmonary comorbidities. The clinical importance of any antiviral active against influenza A is large — the only widely deployed antivirals are the neuraminidase inhibitors (oseltamivir, zanamivir) and the cap-dependent endonuclease inhibitor baloxavir, all of which have meaningful but modest efficacy when started within the first 48 hours of symptoms and increasing resistance issues.

Wu and colleagues at the Beijing University of Chemical Technology reported in 2016 (published in Viruses, MDPI) that quercetin inhibits influenza A virus entry at the hemagglutinin step. Influenza enters host cells through a multi-step process: hemagglutinin on the viral surface binds sialic-acid residues on the host cell, the virus is endocytosed, the endosome acidifies, the acidified hemagglutinin undergoes a conformational change that exposes a fusion peptide, and the fusion peptide inserts into the endosomal membrane to bring the viral and host membranes together for genome release. Wu's group demonstrated that quercetin binds to the HA2 subunit of hemagglutinin and inhibits the acid-triggered conformational change required for membrane fusion. The in-vitro effect was robust across multiple influenza A subtypes including H1N1, H3N2, H5N1, and H7N9 — suggesting the binding site is structurally conserved across HA variants.

Additional mechanisms documented in subsequent in-vitro work: quercetin also inhibits the influenza neuraminidase (the enzymatic target of oseltamivir) at higher concentrations, inhibits influenza RNA polymerase activity through both direct binding and the zinc-ionophore-mediated polymerase inhibition described above, and reduces influenza-induced oxidative stress in lung tissue (limiting the severity of secondary tissue damage that drives much of influenza pneumonia pathology).

The mouse-model evidence is consistent. Multiple groups have shown that oral quercetin pretreatment reduces influenza A viral titers in lung tissue, reduces weight loss, reduces lung pathology, and improves survival in lethal-dose mouse-adapted influenza challenge models. The effect sizes are meaningful (30-60% reductions in viral titer at typical dosing) and reproducible.

Human clinical-trial evidence specific to influenza A is limited. The Nieman 2007 trial in trained cyclists found that quercetin 1000 mg/day reduced incidence of upper-respiratory illness in a three-day post-exercise window when participants were sequestered together — the design captures intensive viral exposure including likely influenza A in some seasons. Larger influenza-specific trials have not been published. The realistic position is that the mechanistic and animal-model evidence is strong and the human evidence is supportive but limited, with quercetin best positioned as an adjunct alongside annual influenza vaccination and standard antiviral therapy rather than as a primary treatment.

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Epstein-Barr Virus — Reactivation Suppression

Epstein-Barr virus (EBV, human herpesvirus 4) is a double-stranded DNA gammaherpesvirus that infects more than 90% of the global adult population. Primary infection in adolescence or young adulthood produces infectious mononucleosis; primary infection in childhood is typically asymptomatic. After primary infection, EBV establishes lifelong latency in memory B cells. Periodic asymptomatic reactivation produces shedding in oropharyngeal secretions and accounts for ongoing transmission. EBV reactivation also contributes to the pathogenesis of several diseases, most notably nasopharyngeal carcinoma (driven by latent EBV genes in epithelial cells), Burkitt's lymphoma, post-transplant lymphoproliferative disorder, and a growing body of evidence implicating EBV in multiple sclerosis (Bjornevik 2022 Science demonstrated that EBV infection precedes MS onset with a hazard ratio of 32 in a large US military cohort).

Chronic EBV reactivation is also a recurring theme in post-viral chronic fatigue syndrome and in some adult patients with persistent fatigue and lymphadenopathy. Suppressing EBV reactivation is therefore a meaningful clinical goal in selected patients with chronic symptomatic reactivation.

The in-vitro evidence for quercetin against EBV is concentrated on the lytic-reactivation phase of the EBV life cycle. Liao and colleagues at the Chinese Academy of Sciences reported in 2010 (published in Antiviral Research) that quercetin and its more potent derivative chrysin suppressed lytic-cycle reactivation in EBV-positive Akata Burkitt lymphoma cells and in nasopharyngeal carcinoma cell lines. The mechanism involves inhibition of the BZLF1 transcription factor (the master switch that triggers the lytic reactivation cascade) and downstream inhibition of EBV DNA polymerase activity. The effect is concentration-dependent in the 5-50 µM range.

Additional in-vitro work has shown that quercetin inhibits EBV-induced cellular transformation of primary B cells and reduces expression of EBV latent membrane protein 1 (LMP1), which is the dominant oncogene in EBV-driven malignancies. The clinical translation is mostly theoretical at this point — there are no large randomized trials of quercetin in EBV-related disease. The mechanistic rationale supports its inclusion in integrative protocols for chronic EBV reactivation in symptomatic adults, typically at 500-1000 mg/day combined with other antiviral nutritional supports (lysine, vitamin C, zinc, monolaurin in some protocols).

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Herpes Simplex Virus (HSV-1 and HSV-2) In Vitro

Herpes simplex virus types 1 and 2 are double-stranded DNA alphaherpesviruses that produce orolabial herpes (HSV-1 in the majority of cases) and genital herpes (HSV-2 in the majority of cases, though HSV-1 increasingly causes primary genital infection in younger populations). Like all herpesviruses, HSV establishes lifelong latency — in sensory nerve ganglia for HSV — with periodic reactivation. Standard antiviral management uses acyclovir, valacyclovir, or famciclovir for both acute outbreaks and chronic suppression.

The in-vitro evidence for quercetin against HSV goes back to the early 1980s. Kaul, Middleton, and Ogra published an influential 1985 paper in the Journal of Medical Virology screening flavonoids against several DNA and RNA viruses, including HSV-1. Quercetin was among the more potent flavonoids, with a 50% inhibitory concentration in the low micromolar range against HSV-1 in cell culture. Subsequent work has confirmed activity against both HSV-1 and HSV-2 with multiple proposed mechanisms: direct inhibition of viral DNA polymerase, interference with the early gene expression cascade, reduced viral protein synthesis, and disrupted virion assembly. Combination experiments with quercetin plus acyclovir have shown additive or modestly synergistic effects in vitro.

Topical formulations of quercetin (and the chemically related flavonoid eriodictyol) have been tested in small clinical series for recurrent oral and genital herpes outbreaks, with reduced lesion duration and reduced viral shedding compared to placebo. These trials are small and underpowered, and topical preparations are not widely commercially available, but the mechanistic and small-trial evidence is consistent enough that integrative practitioners commonly recommend daily quercetin (500-1000 mg/day) combined with daily lysine (1-3 g/day) and daily zinc (15-30 mg/day) for adults with frequent recurrent HSV outbreaks. The combination is an adjunct to standard prescription antiviral therapy, not a replacement for it in patients with severe or frequent outbreaks.

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Hepatitis C — NS3 Protease and Heat-Shock Inhibition

Hepatitis C virus (HCV) is a positive-sense single-stranded RNA flavivirus and a major cause of chronic hepatitis, cirrhosis, and hepatocellular carcinoma globally. The therapeutic landscape was transformed by the direct-acting antiviral drugs (sofosbuvir, ledipasvir, glecaprevir, pibrentasvir, and the broader class of NS3/NS4A protease inhibitors and NS5A and NS5B polymerase inhibitors), which now achieve sustained virologic response in more than 95% of treated patients across genotypes. Quercetin is not a substitute for direct-acting antivirals in patients who can access them — that point should be unambiguous — but the in-vitro work is interesting and the mechanisms documented are biologically clean.

Gonzalez and colleagues at the Hospital del Mar Research Institute in Barcelona reported in 2009 (published in Hepatology) that quercetin inhibits HCV production in cell culture by two distinct mechanisms. First, quercetin inhibits the host-cell heat shock protein chaperones (HSP40 and HSP70) that HCV requires to assemble its replication complex on the cytoplasmic side of the endoplasmic reticulum. Without functional HSP-mediated chaperoning, viral RNA replication and virion assembly are both impaired. Second, quercetin directly inhibits the HCV NS3 protease at higher concentrations, providing a second independent point of antiviral pressure.

A small open-label clinical trial by Lu and colleagues in 2016 (published in Journal of Hepatology) tested oral quercetin in patients with chronic hepatitis C who were not eligible for or were awaiting direct-acting antiviral therapy. The trial showed measurable reductions in plasma HCV RNA levels over 28 days at doses of 1 g/day, with no serious adverse events. The reductions were modest (about 0.5-log) and not clinically curative, but they served as proof-of-principle that the in-vitro antiviral mechanism translates to detectable in-vivo activity in human HCV infection.

The honest framing for HCV-infected patients today: pursue direct-acting antiviral therapy through standard hepatology care if at all possible — the cure rates are nearly universal and the duration is short. Quercetin is at most a supportive adjunct for patients awaiting therapy or for patients in settings where direct-acting antivirals are not accessible. The hepatoprotective effect of quercetin (separate from its antiviral mechanism) may also support liver function in patients with chronic HCV-related fibrosis through its broader antioxidant and anti-inflammatory activities.

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Dosing — The Quercetin + Zinc + Vitamin C Stack

The classical integrative-medicine protocol for upper-respiratory infection prophylaxis and early treatment combines quercetin, zinc, and vitamin C, each chosen for a complementary mechanism:

Typical dosing for each component:

The zinc dose for chronic daily use should not exceed 30-40 mg/day of elemental zinc without copper co-supplementation — chronic high-dose zinc induces metallothionein in enterocytes, which then preferentially binds copper from the diet and produces copper deficiency over months. The relevant co-supplementation is 1-2 mg/day of elemental copper (typically as copper bisglycinate or copper sebacate) for every 15 mg of supplemental zinc above the dietary baseline. The high-dose zinc lozenge protocol for acute illness is too short (less than 7 days) to produce copper depletion and does not require copper co-supplementation.

The vitamin C dose tolerable in divided doses is substantially higher than the dose tolerable in a single bolus — spreading 2 g/day across four 500 mg doses is well tolerated whereas a single 2 g bolus causes osmotic gastrointestinal upset in most adults. See our Vitamin C page for the broader context.

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Bioavailability — Phytosome vs Raw Aglycone

The pharmacokinetic profile of oral quercetin is a major determinant of whether the doses recommended above actually achieve the intracellular concentrations needed for the in-vitro mechanisms to translate into in-vivo effects. Free quercetin aglycone (the form found in raw supplements and in plant foods after acid hydrolysis) is highly hydrophobic, with aqueous solubility of less than 1 mg/L, and is absorbed poorly — oral bioavailability of free aglycone is in the low single-digit percent range. Most ingested quercetin is excreted unmetabolized in feces or undergoes microbial degradation in the colon to phenolic acid metabolites that lack the parent compound's antiviral activity.

Several formulations address this absorption problem:

The practical choice depends on the use case. For daily prophylactic use in healthy adults at moderate cost, standard quercetin + bromelain at 500 mg twice daily is reasonable and inexpensive. For chronic conditions where higher tissue exposure matters (MCAS as discussed on the Allergy and Histamine page, chronic viral reactivation, senolytic dosing as discussed on the Senolytic Activity page), the phytosome formulation is worth the additional cost. Taking standard quercetin with a fat-containing meal modestly improves absorption and is sensible regardless of formulation.

An important point about dietary sources: the quercetin in plant foods (apples, onions, capers, kale, dark berries) is mostly in glycoside forms (quercetin-3-rutinoside, quercetin-3-glucoside) that are better absorbed than free aglycone. Dietary quercetin from onions and apples is therefore not negligible, although the absolute amounts (typically 10-30 mg/day from a varied diet) are well below the doses used in supplement protocols.

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Cautions and Drug Interactions

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Key Research Papers

  1. Dabbagh-Bazarbachi H, Clergeaud G, Quesada IM, Ortiz M, O'Sullivan CK, Fernandez-Larrea JB (2014). Zinc ionophore activity of quercetin and epigallocatechin-gallate: from Hepa 1-6 cells to a liposome model. Journal of Agricultural and Food Chemistry. — PubMed
  2. te Velthuis AJW, van den Worm SHE, Sims AC, Baric RS, Snijder EJ, van Hemert MJ (2010). Zn(2+) inhibits coronavirus and arterivirus RNA polymerase activity in vitro and zinc ionophores block the replication of these viruses in cell culture. PLoS Pathogens. — PubMed
  3. Hemilä H (2017). Zinc lozenges and the common cold: a meta-analysis comparing zinc acetate and zinc gluconate, and the role of zinc dosage. JRSM Open. — PubMed
  4. Ganesan S, Faris AN, Comstock AT, et al. (2012). Quercetin inhibits rhinovirus replication in vitro and in vivo. Antiviral Research. — PubMed
  5. Wu W, Li R, Li X, He J, Jiang S, Liu S, Yang J (2016). Quercetin as an antiviral agent inhibits influenza A virus (IAV) entry. Viruses. — PubMed
  6. Heinz SA, Henson DA, Austin MD, Jin F, Nieman DC (2010). Quercetin supplementation and upper respiratory tract infection: a randomized community clinical trial. Pharmacological Research. — PubMed
  7. Nieman DC, Henson DA, Gross SJ, et al. (2007). Quercetin reduces illness but not immune perturbations after intensive exercise. Medicine and Science in Sports and Exercise. — PubMed
  8. Liao TJ, Liu CW, Wu WT, Hsu MJ (2010). Effect of quercetin on Epstein-Barr virus lytic-cycle reactivation in Akata cells. Antiviral Research. — PubMed
  9. Kaul TN, Middleton E Jr, Ogra PL (1985). Antiviral effect of flavonoids on human viruses. Journal of Medical Virology. — PubMed
  10. Gonzalez O, Fontanes V, Raychaudhuri S, et al. (2009). The heat shock protein inhibitor quercetin attenuates hepatitis C virus production. Hepatology. — PubMed
  11. Lu NT, Crespi CM, Liu NM, et al. (2016). A phase I dose escalation study demonstrates quercetin safety and explores potential for bioflavonoid antivirals in patients with chronic hepatitis C. Phytotherapy Research. — PubMed
  12. Riva A, Ronchi M, Petrangolini G, Bosisio S, Allegrini P (2019). Improved oral absorption of quercetin from Quercetin Phytosome, a new delivery system based on food-grade lecithin. European Journal of Drug Metabolism and Pharmacokinetics. — PubMed
  13. Egert S, Wolffram S, Bosy-Westphal A, et al. (2008). Daily quercetin supplementation dose-dependently increases plasma quercetin concentrations in healthy humans. Journal of Nutrition. — PubMed
  14. Bjornevik K, Cortese M, Healy BC, et al. (2022). Longitudinal analysis reveals high prevalence of Epstein-Barr virus associated with multiple sclerosis. Science. — PubMed

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