Clinical Immunology · STAT1 Gain-of-Function Syndrome

Understanding Genetic and Functional Immune Tests

At a Glance

STAT1-GOF diagnosis relies on a compatible clinical picture plus STAT1 genetic testing; a pathogenic variant may be enough even when functional testing is unavailable. Normal antibody levels or vaccine responses do not rule it out, so pSTAT1, Th17 results, and expert review may add context.

Because STAT1-GOF Syndrome is a complex disorder where the immune system is both overactive and under-defensive, standard blood tests often come back looking perfectly “normal.” Reaching a diagnosis requires specialized genetic and functional testing to see how the immune system is actually behaving at a molecular level ^[1]^[2].

The Diagnostic Approach: Genetic and Functional Testing

To reach a diagnosis, doctors rely on clinical assessment paired with genetic and sometimes functional testing ^[1]^[3]. A well-established pathogenic STAT1 variant and compatible clinical picture may be sufficient even when a functional assay is unavailable.

Genetic Sequencing: This test reads the DNA of the STAT1 gene. It usually identifies a heterozygous variant, meaning the change is only on one of the two copies of the gene ^[4]. Most cases are autosomal dominant, which your doctor may confirm by testing parents to see if the change was inherited or occurred for the first time in the patient (de novo) ^[1]^[5].
Functional Phospho-STAT1 (pSTAT1) Assays: Finding a genetic change isn’t always enough, because some changes are Variants of Uncertain Significance (VUS). A pSTAT1 assay is a specialized test where your blood cells are exposed to immune “alarms” (like interferons) in a lab. In STAT1-GOF, the cells typically show an abnormally high or prolonged “on” signal, known as hyperphosphorylation ^[1]^[3]^[6]. A VUS cannot be classified by one test alone; these tests are supportive and expert-interpreted.

Why “Normal” Results Can Be Misleading

One of the most frustrating parts of the diagnostic journey is being told that tests are normal while symptoms continue. In STAT1-GOF, your immune system often produces the right amount of defenses, but they don’t work correctly.

Immunoglobulins (IgG/IgA/IgM): Most patients with STAT1-GOF have normal or even high levels of antibodies (immunoglobulins) ^[7]^[8].
Vaccine Responses: You may have a normal response to standard childhood vaccines, yet still be highly vulnerable to specific types of fungal or bacterial infections ^[9]^[10].
A normal result on these standard tests does not rule out STAT1-GOF ^[8]^[9].

Key Technical Terms to Know

Heterozygous: Having a genetic change on only one copy of a gene pair ^[4].
Gain-of-Function (GOF): A mutation that makes a protein hyperactive or gives it a new, altered “job” ^[3].
Phosphorylation: A chemical “on” switch for proteins. In this syndrome, the STAT1 protein gets “switched on” too easily or stays on too long ^[1]^[11].
Dephosphorylation: The “off” switch that resets the protein. Some patients have “delayed dephosphorylation,” meaning their immune signal won’t turn off properly ^[6]^[12].
Th17 Cells: A specialized group of white blood cells that fight fungi. While 82-88% of STAT1-GOF patients in cohorts have low levels of these cells, Th17 enumeration varies by age and laboratory method ^[13]^[14]. A normal or low result does not independently confirm or exclude the condition.
Flow Cytometry: A laboratory technique used to count and analyze the characteristics of individual cells, such as measuring pSTAT1 signals or Th17 cell counts ^[1]^[15].

Lab Report Completeness Checklist

When you receive your lab results, ensure they contain these critical details. If any are missing, ask your specialist for the full report.

[ ] Specific Variant Details: The report should list the exact location of the change (e.g., c.1078G>C) and the protein change (e.g., p.Val360Leu) ^[9]^[16].
[ ] Zygosity: It should state “Heterozygous” ^[4].
[ ] Variant Classification: Ideally labeled as “Pathogenic” or “Likely Pathogenic.” If it says “Variant of Uncertain Significance” (VUS), functional testing and expert evaluation is critical ^[17]^[18].
[ ] pSTAT1 Results: Documentation of how the cells responded to IFN-alpha or IFN-gamma compared to a healthy control ^[1]^[6].
[ ] Th17 Quantification: A count of Th17 cells or “CCR6+ CD4+” cells ^[13]^[15].
[ ] Allele Fraction: In most cases, this will be around 50%, which can support a heterozygous germline variant in the tested sample, though it does not prove every cell carries it or establish gain-of-function behavior ^[16].

Having this complete picture allows your care team to tailor treatments—like JAK inhibitors—that address the hyperactive signals found in your tests ^[19].

Common questions in this guide

How is STAT1-GOF syndrome diagnosed?

Diagnosis combines a compatible pattern of illness with genetic testing of STAT1. A clearly disease-causing variant may be enough, while pSTAT1 testing, Th17 measurement, and other functional studies can provide support, especially when the genetic result is uncertain.

Can I have STAT1-GOF if my immunoglobulin levels are normal?

Yes. People with STAT1-GOF often have normal or high IgG, IgA, and IgM levels and may respond normally to routine vaccines, so these results do not rule out the syndrome.

What does a heterozygous STAT1 variant mean?

It means the variant is present in one of the two copies of the STAT1 gene. STAT1-GOF is usually autosomal dominant, so the change may be inherited from a parent or may arise de novo, meaning it is new in the patient.

What does a pSTAT1 test show?

The test exposes blood cells to interferons and measures how strongly and how long the STAT1 signal is activated. Excessive activation, called hyperphosphorylation, or delayed shutoff can support a gain-of-function result, but one test alone does not prove that a VUS causes disease.

Why is Th17 cell testing done in STAT1-GOF?

Th17 cells help protect against fungal infections, and many people with STAT1-GOF have low levels. Results vary with age and laboratory method, so a normal or low Th17 count alone cannot confirm or exclude the syndrome.

What should be included in a STAT1 genetic test report?

The report should identify the exact DNA and protein change, whether it is heterozygous, and whether it is classified as pathogenic, likely pathogenic, or a VUS. It may also include pSTAT1 responses, Th17 counts, and allele fraction, which help specialists interpret the result in context.

Get notified when new evidence is published on STAT1-related autoimmune enteropathy and endocrinopathy-susceptibility to chronic infections syndrome.

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References (19)

1
The Extended Clinical Phenotype of 26 Patients with Chronic Mucocutaneous Candidiasis due to Gain-of-Function Mutations in STAT1.
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2
Recurrent, Severe Aphthous Stomatitis and Mucosal Ulcers as Primary Manifestations of a Novel STAT1 Gain-of-Function Mutation.
Erdős M, Jakobicz E, Soltész B, et al.
Frontiers in immunology 2020; (11()):967 doi:10.3389/fimmu.2020.00967.
PMID: 32547544
3
Human STAT1 Gain-of-Function Heterozygous Mutations: Chronic Mucocutaneous Candidiasis and Type I Interferonopathy.
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Journal of clinical immunology 2020; (40(8)):1065-1081 doi:10.1007/s10875-020-00847-x.
PMID: 32852681
4
A novel gain-of-function STAT1 mutation resulting in basal phosphorylation of STAT1 and increased distal IFN-γ-mediated responses in chronic mucocutaneous candidiasis.
Martinez-Martinez L, Martinez-Saavedra MT, Fuentes-Prior P, et al.
Molecular immunology 2015; (68(2 Pt C)):597-605.
PMID: 26514428
5
Bronchiectasis and Recurrent Respiratory Infections with a De Novo STAT1 Gain-of-Function Variant: First Case in Korea.
Huh HJ, Jhun BW, Choi SR, et al.
Yonsei medical journal 2018; (59(8)):1004-1007 doi:10.3349/ymj.2018.59.8.1004.
PMID: 30187709
6
STAT1 Gain-of-Function Mutations Cause High Total STAT1 Levels With Normal Dephosphorylation.
Zimmerman O, Olbrich P, Freeman AF, et al.
Frontiers in immunology 2019; (10()):1433 doi:10.3389/fimmu.2019.01433.
PMID: 31354696
7
Impaired STAT3-Dependent Upregulation of IL2Rα in B Cells of a Patient With a STAT1 Gain-of-Function Mutation.
van Zelm MC, Bosco JJ, Aui PM, et al.
Frontiers in immunology 2019; (10()):768 doi:10.3389/fimmu.2019.00768.
PMID: 31068927
8
Qualitative Immunoglobulin Deficiency Causes Bacterial Infections in Patients with STAT1 Gain-of-Function Mutations.
Chen R, Mu H, Chen X, et al.
Journal of clinical immunology 2024; (44(5)):124 doi:10.1007/s10875-024-01720-x.
PMID: 38758476
9
Disseminated Tuberculosis and Early-Onset SLE in a Child with a Novel STAT1 Gain-of-Function Mutation: a Case Report.
Arslan S, Sert A, Özçelik M, et al.
Journal of clinical immunology 2026; (46(1)).
PMID: 42319627
10
A gain-of-function mutation of STAT1: A novel genetic factor contributing to chronic mucocutaneous candidiasis.
Eslami N, Tavakol M, Mesdaghi M, et al.
Acta microbiologica et immunologica Hungarica 2017; (64(2)):191-201 doi:10.1556/030.64.2017.014.
PMID: 28597685
11
Identification of a distinct subset of disease-associated gain-of-function missense mutations in the STAT1 coiled-coil domain as system mutants.
Petersen J, Staab J, Bader O, et al.
Molecular immunology 2019; (114()):30-40 doi:10.1016/j.molimm.2019.07.008.
PMID: 31336247
12
Live Cell Imaging Demonstrates Multiple Routes Toward a STAT1 Gain-of-Function Phenotype.
Giovannozzi S, Lemmens V, Hendrix J, et al.
Frontiers in immunology 2020; (11()):1114 doi:10.3389/fimmu.2020.01114.
PMID: 32582194
13
Clinical Relevance of Gain- and Loss-of-Function Germline Mutations in STAT1: A Systematic Review.
Zhang W, Chen X, Gao G, et al.
Frontiers in immunology 2021; (12()):654406 doi:10.3389/fimmu.2021.654406.
PMID: 33777053
14
Heterozygous STAT1 gain-of-function mutations underlie an unexpectedly broad clinical phenotype.
Toubiana J, Okada S, Hiller J, et al.
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PMID: 27114460
15
Chronic mucocutaneous candidiasis: characterization of a family with STAT-1 gain-of-function and development of an ex-vivo assay for Th17 deficiency of diagnostic utility.
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PMID: 26621323
16
A Child with Chronic Mucocutaneous Candidiasis Harbors a Novel Gain-of-Function Mutation in STAT1.
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PMID: 41026300
17
Alanine-scanning mutagenesis of human signal transducer and activator of transcription 1 to estimate loss- or gain-of-function variants.
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18
Biallelic TRAF3IP2 variants causing chronic mucocutaneous candidiasis in a child harboring a STAT1 variant.
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19
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PMID: 35486339

This page is for informational purposes only and does not constitute medical advice. An immunologist or genetics specialist should interpret your STAT1-GOF results in the context of your symptoms and family history.