rs368234815 — IFNL4 ss469415590 (TT/ΔG)

Causal frameshift polymorphism controlling IFNL4 protein production; the ΔG allele creates functional interferon lambda 4 which paradoxically impairs hepatitis C clearance

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IFNL4 — The Interferon That Backfires Against Hepatitis C

Interferon lambda 411 Interferon lambda 4
A newly discovered type III interferon, encoded by IFNL4 on chromosome 19q13.13, identified in 2013 as the causal gene behind the well-known "IL28B" hepatitis C association is one of biology's great counterintuitive discoveries: a protein that belongs to the antiviral interferon family yet actively impairs the body's ability to clear a specific virus — hepatitis C (HCV). The rs368234815 polymorphism (originally designated ss469415590) is the molecular switch that controls whether you can produce IFN-λ4 at all. If you carry the ΔG allele, your cells make functional IFN-λ4 and your chances of clearing HCV spontaneously or responding to treatment are significantly reduced. If you are homozygous TT, the gene is broken by a frameshift, IFN-λ4 is never produced, and your immune system is paradoxically better at eliminating HCV.

This is not a rare clinical curiosity. The ΔG allele is the ancestral form — humans evolved with IFN-λ4 as a functional gene. The TT null variant arose and was then driven to high frequency by positive selection22 driven to high frequency by positive selection
especially strong in East Asia, where TT frequencies approach 90%, suggesting a survival advantage during periods of HCV or other viral epidemics. Among people of African ancestry, up to 80% still carry the ancestral ΔG allele. This variant supersedes the previously used rs12979860 (the "IL28B" SNP) as the direct functional test for IFNL4 activity.

The Mechanism

The rs368234815 polymorphism is a dinucleotide indel33 dinucleotide indel
A two-nucleotide insertion/deletion; the ΔG allele involves deletion of one thymine, shifting the reading frame in exon 1 of IFNL4. The ΔG allele creates an intact open reading frame, allowing translation of the full IFNL4 protein. The TT allele introduces a frameshift that generates a premature stop codon, producing only truncated, non-functional polypeptides. In this way, TT homozygotes are complete IFNL4 "knockouts" at the population level.

The paradox is that recombinant IFN-λ4 protein has real antiviral activity against HCV in cell culture — it signals through the IFN-λ receptor and activates hundreds of interferon-stimulated genes44 interferon-stimulated genes
ISGs, whose protein products directly inhibit viral replication. Why, then, does IFN-λ4 production predict worse outcomes? Two mechanisms have emerged. First, IFN-λ4 is largely retained in the endoplasmic reticulum55 endoplasmic reticulum
ER; only small amounts are secreted extracellularly, where it induces ER stress. ER-stressed cells are significantly weaker activators of HCV-specific CD8+ T cells, blunting the adaptive immune response that is ultimately required for viral clearance. Second, the chronically elevated ISG expression driven by IFN-λ4 may induce a state of "interferon exhaustion" in the liver — when hepatocytes are pre-saturated with interferon signaling, exogenous peginterferon therapy cannot drive ISG expression higher, and the additional antiviral effect is lost. Recent work also shows that IFN-λ4 impairs HCV antigen presentation to T cells66 IFN-λ4 impairs HCV antigen presentation to T cells
potentially allowing virus to persist by avoiding immune recognition, suggesting a third mechanism by which functional IFN-λ4 undermines clearance despite appearing antiviral in isolation.

The Evidence

The original 2013 discovery by Prokunina-Olsson et al.77 Prokunina-Olsson et al.
"A variant upstream of IFNL3 (IL28B) creating a new interferon gene IFNL4 is associated with impaired clearance of hepatitis C virus" used RNA-sequencing in primary human hepatocytes stimulated with double-stranded RNA to mimic HCV infection. The study found that rs368234815 ΔG carriers could be distinguished from TT individuals by a new induced transcript upstream of IFNL388 new induced transcript upstream of IFNL3
the newly discovered IFNL4 gene, and that rs368234815 predicted HCV clearance more accurately than the then-standard rs12979860 marker.

For treatment outcomes, a meta-analysis spanning multiple HCV genotypes, ethnicities, and treatment regimens99 meta-analysis spanning multiple HCV genotypes, ethnicities, and treatment regimens
including both peginterferon/ribavirin and direct-acting antivirals confirmed that TT/TT individuals achieve higher sustained virologic response (SVR) rates regardless of therapy. In DAA-treated HCV genotype 11010 DAA-treated HCV genotype 1
sofosbuvir-based regimens, SVR was 98.6% for TT/TT versus 86.8% for ΔG/ΔG (P=0.01) — a clinically meaningful difference even in the era of highly effective antivirals. For spontaneous clearance in hemodialysis patients1111 hemodialysis patients
a cohort where HCV exposure rates are high, TT/TT carriers had 6.38-fold higher odds of clearing HCV without treatment (OR 6.38, 95% CI 1.69–24.2, P=0.003).

The variant is also relevant for HCV genotype 4 patients treated with sofosbuvir plus ribavirin, for IFN-based regimens in HIV/HCV coinfection, and for predicting post-transplant fibrosis progression in HCV-positive liver transplant recipients. Beyond HCV, emerging data suggest IFNL4 genotype modestly influences HIV seroconversion rates (ΔG/ΔG carriers may have slightly lower HIV acquisition odds) and is associated with prostate cancer risk in men with high sexually-transmitted infection exposure — though these applications remain investigational.

Practical Implications

For the vast majority of people who have never been infected with HCV, this variant's clinical significance is limited. HCV is not transmitted through casual contact — it requires blood-to-blood exposure (sharing needles, unscreened blood transfusions before 1992, certain medical procedures in under-resourced settings, or rarely sexual transmission). If you have no HCV risk factors, carry no HCV infection, and have no family history of liver disease from HCV, this variant is informationally interesting but requires no immediate action.

If you have been diagnosed with chronic hepatitis C or are at elevated risk (healthcare workers, people who inject drugs, those who received blood products before 1992), your IFNL4 genotype directly informs treatment planning. ΔG carriers — especially ΔG/ΔG homozygotes — should discuss with their hepatologist whether a standard 8-week DAA course is sufficient or whether extended therapy is warranted. Several clinical studies support extended treatment duration for ΔG/ΔG individuals receiving sofosbuvir-based regimens.

Interactions

rs368234815 and rs12979860 are in strong linkage disequilibrium (LD) and measure the same biological effect — but rs368234815 is the direct functional variant. In African Americans, where LD between the two variants is lower, rs368234815 is a substantially better predictor. If you have been genotyped with the older rs12979860 (C/T) assay, the favorable C allele at rs12979860 corresponds to the TT allele at rs368234815; the unfavorable T allele at rs12979860 corresponds to the ΔG allele. The rs117648444 Pro70Ser variant1212 rs117648444 Pro70Ser variant
a coding variant in IFNL4 that modulates IFN-λ4 protein activity is a secondary modifier among ΔG carriers — those carrying the S70 form of IFN-λ4 have impaired protein function and better treatment outcomes than P70 ΔG carriers, providing additional prognostic resolution within the heterozygous group.

Drug Interactions

peginterferon alfa/ribavirin reduced_efficacy literature
sofosbuvir-based DAA regimens reduced_efficacy literature

Genotype Interpretations

What each possible genotype means for this variant.

TT No IFNL4 Production Beneficial

Optimal HCV clearance genetics — IFNL4 gene non-functional

You carry two copies of the TT allele, meaning your IFNL4 gene is non-functional due to a frameshift mutation. Your cells cannot produce interferon lambda 4 protein. This is, paradoxically, the favorable genotype for hepatitis C: people with TT/TT are the best spontaneous clearers of HCV and achieve the highest sustained virologic response rates to both interferon-based and direct-acting antiviral therapies. About 49% of the global population (and approximately 44% of Europeans, rising to over 85% in East Asians) shares this genotype. The TT allele appears to have been favored by natural selection in populations outside Africa, likely because it improves outcomes with ancient RNA virus infections.

DT One ΔG Allele Intermediate

Heterozygous ΔG — partial IFNL4 production, modestly reduced HCV clearance

The intermediate phenotype in ΔG/TT individuals reflects gene dosage: one functional IFNL4 allele produces enough IFN-λ4 to elevate pre-treatment ISG expression in the liver, which correlates with blunted response to exogenous interferon therapy. For DAA regimens (sofosbuvir, ledipasvir, velpatasvir combinations), the influence of IFNL4 genotype is smaller than with older peginterferon-based treatments, but remains statistically and clinically detectable. The secondary modifier rs117648444 (Pro70Ser in the IFNL4 protein) further stratifies ΔG/TT individuals: those with the S70 form have lower IFN-λ4 activity and approach TT/TT outcomes.

DD Homozygous ΔG High Risk

Homozygous ΔG — full IFNL4 production, significantly impaired HCV clearance

In ΔG/ΔG individuals, IFN-λ4 is produced at maximum levels when viral RNA triggers the innate immune response. This causes chronically elevated interferon-stimulated gene expression in the liver — a state sometimes called "interferon saturation" — which paradoxically leaves little room for additional antiviral response when exogenous interferon therapy is administered. Three mechanisms have been proposed: (1) IFN-λ4 retained in the endoplasmic reticulum induces ER stress, weakening HCV-specific CD8+ T cell responses; (2) chronic ISG pre-activation creates a refractory state for exogenous interferon; and (3) IFN-λ4 directly impairs HCV antigen presentation. DAA therapy (which acts directly on viral replication machinery rather than boosting host immunity) partially overcomes the IFNL4 disadvantage, but ΔG/ΔG individuals still achieve meaningfully lower SVR rates even with modern agents. Extended treatment duration — 12 versus 8 weeks for some sofosbuvir regimens — is supported by available data for this genotype.

This genotype carries no implications for health conditions unrelated to HCV exposure. The ΔG allele is the ancestral human variant; having it does not represent a disease state.