# FOXP3

**FOXP3** (forkhead box P3) is a transcription factor encoded on the [X chromosome](https://www.edgepedia.org/x-chromosome) that is required for the development and immunosuppressive function of regulatory T (Treg) cells, the CD4+ T cell population that prevents autoimmunity.<sup>[1](https://medlineplus.gov/download/genetics/gene/foxp3.pdf)</sup><sup> • </sup><sup>[2](https://pubmed.ncbi.nlm.nih.gov/25047417/)</sup> The protein binds specific DNA regions and controls the activity of genes involved in regulating the immune system, and it is found primarily in the thymus, where regulatory T cells are produced.<sup>[1](https://medlineplus.gov/download/genetics/gene/foxp3.pdf)</sup> Mutations in FOXP3 cause the severe X-linked autoimmune disease IPEX syndrome in humans and the scurfy phenotype in mice, and the gene's discovery linked the genetics of immune tolerance to regulatory [T cell](https://www.edgepedia.org/t-cell) biology, work recognized by the 2025 [Nobel Prize in Physiology or Medicine](https://www.edgepedia.org/nobel-prize-in-physiology-or-medicine).<sup>[1](https://medlineplus.gov/download/genetics/gene/foxp3.pdf)</sup><sup> • </sup><sup>[3](https://www.livescience.com/health/nobel-prize-in-medicine-goes-to-trio-for-their-work-on-immune-tolerance)</sup>

| Key fact | Detail |
|---|---|
| Gene location | Xp11.23 on the X chromosome; 12 exons, the first noncoding and the remaining 11 coding<sup>[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC10588449/)</sup> |
| Protein size | Full-length human FOXP3 is 431 amino acids, about 47.25 kDa<sup>[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC10588449/)</sup> |
| Protein domains | From the N terminus: a repressor domain, a zinc finger, a leucine zipper motif, and a forkhead DNA-binding domain<sup>[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC10588449/)</sup> |
| Function | Requisite transcription factor for development and maintenance of immunosuppressive Treg cell function and immune homeostasis<sup>[2](https://pubmed.ncbi.nlm.nih.gov/25047417/)</sup> |
| Disease | More than 60 FOXP3 mutations cause IPEX syndrome, a rare condition with multiple autoimmune disorders affecting intestines, skin, and endocrine glands<sup>[1](https://medlineplus.gov/download/genetics/gene/foxp3.pdf)</sup> |
| Target genes | Genome-wide analyses indicate FoxP3 binds perhaps as many as ~700 genes and miRNA, involved in TCR signaling, cell communication, and transcriptional regulation<sup>[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC7008219/)</sup> |

## Discovery

The scurfy mutation arose spontaneously in mice at [Oak Ridge National Laboratory](https://www.edgepedia.org/oak-ridge-national-laboratory) in the United States, and early work established it as a fatal, X-linked lymphoreticular disease requiring T cells that mature in a scurfy thymic environment.<sup>[6](https://www.nature.com/articles/nri3650)</sup> The molecular cloning of the FOXP3 gene was reported in 2001, when [Mary E. Brunkow](https://www.edgepedia.org/mary-e-brunkow) and colleagues showed that disruption of a new forkhead/winged-helix protein, named scurfin, causes the fatal lymphoproliferative disorder of the scurfy mouse.<sup>[6](https://www.nature.com/articles/nri3650)</sup> In the same year, [Charles L. Bennett](https://www.edgepedia.org/charles-l-bennett) and colleagues and R. S. Wildin and colleagues showed that mutations of FOXP3 cause IPEX in humans and that X-linked neonatal diabetes mellitus, enteropathy and endocrinopathy syndrome is the human equivalent of mouse scurfy.<sup>[6](https://www.nature.com/articles/nri3650)</sup>

The connection to regulatory T cells followed in 2003. According to a Live Science account of the 2025 Nobel Prize, Brunkow and Ramsdell's contribution was the discovery that some mice carried a gene mutation, named Foxp3, that makes them especially vulnerable to autoimmune diseases, and that alterations to the human version of the gene cause IPEX syndrome; in 2003 [Shimon Sakaguchi](https://www.edgepedia.org/shimon-sakaguchi) demonstrated that the Foxp3 gene governs the development of regulatory T cells.<sup>[3](https://www.livescience.com/health/nobel-prize-in-medicine-goes-to-trio-for-their-work-on-immune-tolerance)</sup> Three 2003 papers, by Sakaguchi's group (Hori, Nomura, and Sakaguchi in *Science*), by Fontenot, Gavin, and Rudensky, and by Khattri, Cox, Yasayko, and Ramsdell, established that Foxp3 programs the development and function of CD4+CD25+ regulatory T cells.<sup>[6](https://www.nature.com/articles/nri3650)</sup> The 2025 Nobel Prize in Physiology or Medicine was awarded for work on immune tolerance, including this linking of FOXP3, the IPEX/scurfy gene, and regulatory T cells.<sup>[3](https://www.livescience.com/health/nobel-prize-in-medicine-goes-to-trio-for-their-work-on-immune-tolerance)</sup>

## Role in regulatory T cells

FOXP3 is the lineage-defining transcription factor of Treg cells. Its gene mutations lead to autoimmune disease in both mice and humans, and Foxp3 expression is used to identify Treg cells: as Foxp3+ T cells in mice, and as Foxp3^high CD45RA− or Foxp3+CD127−CD25^high T cells in humans.<sup>[7](https://www.frontiersin.org/journals/immunology/articles/10.3389/fimmu.2019.03136/full)</sup> T cell receptor (TCR) signaling drives Foxp3 transcription, and in self-reactive thymocytes it is sustained TCR signaling that switches on Foxp3 expression; once the gene is active, a self-reinforcing transcriptional loop maintains Foxp3 levels, securing Treg differentiation and enabling suppressive activity.<sup>[7](https://www.frontiersin.org/journals/immunology/articles/10.3389/fimmu.2019.03136/full)</sup>

**Not a complete program.** Foxp3 can drive Treg cell development and function, but gene expression analysis of Foxp3+ and Foxp3− T cells shows that many Treg cell-specific genes are independent of Foxp3, and Foxp3 expression alone is not sufficient to convert non-Treg cells into Treg cells with a classical Treg-type gene signature and function.<sup>[7](https://www.frontiersin.org/journals/immunology/articles/10.3389/fimmu.2019.03136/full)</sup> [Chromatin immunoprecipitation](https://www.edgepedia.org/chromatin-immunoprecipitation) combined with expression array analysis has identified DNA sequences directly bound by Foxp3 that act as transcriptional activators of some genes and repressors of others.<sup>[7](https://www.frontiersin.org/journals/immunology/articles/10.3389/fimmu.2019.03136/full)</sup> Enforced expression of FoxP3 in human leukemic Jurkat T cells induces a Treg phenotype, including increased expression of Treg-associated surface markers and inhibition of cytokine production.<sup>[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC7008219/)</sup>

## Protein structure and isoforms

The full-length human FOXP3 protein has 431 amino acids and a molecular weight of about 47.25 kDa, with four main functional domains: from the N terminus, a repressor domain, a zinc finger, a leucine zipper motif, and a forkhead [DNA-binding domain](https://www.edgepedia.org/dna-binding-domain).<sup>[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC10588449/)</sup> Structural work has described a domain-swapped FOXP3 dimer on DNA.<sup>[6](https://www.nature.com/articles/nri3650)</sup>

Two isoforms predominate in healthy human natural Treg cells: the full-length isoform (FOXP3^FL) and a shorter isoform produced by transcripts lacking exon 2 (FOXP3ΔE2).<sup>[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC10588449/)</sup> AlphaFold2 predictions and in vitro experiments indicate that the N-terminal domain including exon 2 inhibits the DNA-binding ability of the forkhead domain, which may serve as an auto-inhibitory feedback mechanism.<sup>[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC10588449/)</sup> Known binding partners of FOXP3 exon 2 include RORγt, RORα, and hnRNPF, while exon 7 partners include FOXP1, FOXP3 itself, and DNA.<sup>[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC12477319/)</sup>

## Regulation of FOXP3 expression and stability

Stable Foxp3 expression depends on epigenetic modification. CNS2 stays accessible because the CpG islands within it, called the Treg cell-specific demethylated region (TSDR), lose their methylation, and this unmethylated state can be read as a sign of how stable FOXP3 expression and the Treg phenotype are.<sup>[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC10588449/)</sup> During inflammation, Treg cells may lose their phenotypic properties and convert into effector T cells secondary to alteration of Foxp3 expression and stability.<sup>[7](https://www.frontiersin.org/journals/immunology/articles/10.3389/fimmu.2019.03136/full)</sup>

**Post-translational control.** Networks of post-translational modifications control FOXP3, including regulation of DNA binding, transactivation potential, and proteasomal degradation.<sup>[2](https://pubmed.ncbi.nlm.nih.gov/25047417/)</sup> Acetylation of FoxP3 upon complexing with p300 allows it to evade proteasomal degradation, a post-translational mechanism of FoxP3 stability.<sup>[9](https://academic.oup.com/cei/article/197/1/36/6402553)</sup> Acetylation may prevent polyubiquitination on the same lysine residue through a competition-based mechanism, and the acetylation of TIP60 functions by acting with p300 as a molecular switch that changes TIP60 binding partners.<sup>[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC7008219/)</sup> Sirt1-deficient induced Treg cells display increased FoxP3 stability and restrained conversion into pathogenic T cells, and HDAC inhibitor therapy increases acetylation of both histones and the forkhead domain of FoxP3, along with FoxP3 protein expression and DNA binding.<sup>[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC7008219/)</sup> Abrogating the FoxP3–TIP60 interaction, a defect caused by the germline foxp3^A384T IPEX mutation, relieves human Treg cell suppressive capacity while retaining major aspects of the Treg phenotype, including repression of proliferation and inflammatory cytokines.<sup>[9](https://academic.oup.com/cei/article/197/1/36/6402553)</sup>

## IPEX syndrome and clinical relevance

Hemizygous (Having only one copy of a gene, as X-linked genes in males) mutations in FOXP3 cause immune dysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX) syndrome, a rare condition characterized by multiple autoimmune disorders typically affecting the intestines, skin, and endocrine glands.<sup>[1](https://medlineplus.gov/download/genetics/gene/foxp3.pdf)</sup><sup> • </sup><sup>[10](https://pmc.ncbi.nlm.nih.gov/articles/PMC12540517/pdf/10875_2025_Article_1934.pdf)</sup> Most mutations change a single amino acid in the DNA-binding region of the protein or lead to an abnormally short, nonfunctional protein; without functional Treg cells the body cannot control immune responses, and normal tissues are attacked.<sup>[1](https://medlineplus.gov/download/genetics/gene/foxp3.pdf)</sup> Because FOXP3 expression can remain preserved despite hemizygous missense mutations, flow cytometric detection of FOXP3 alone does not rule out IPEX syndrome.<sup>[10](https://pmc.ncbi.nlm.nih.gov/articles/PMC12540517/pdf/10875_2025_Article_1934.pdf)</sup> Clinical severity can vary even among carriers of the same mutation: one missense mutation (F324L) between the leucine zipper and forkhead domains, which did not impede protein expression or transcriptional activity, was associated with mild or no disease and a partial defect in suppressive activity, and two brothers with this mutation showed different clinical presentations.<sup>[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC1464917/)</sup>

FOXP3 biology also informs therapy. Treg cells isolated from patient blood as CD4+CD25+CD127^low FoxP3+ cells are being tested as cellular therapies in settings including hematopoietic stem cell transplantation, solid organ transplantation, graft-versus-host disease, autoimmune diabetes, multiple sclerosis, and autoimmune arthritis, and chimeric antigen receptor engineering has been applied to Tregs targeting autoimmunity drivers such as the insulin β chain 10–23 peptide in autoimmune diabetes and myelin oligodendrocyte glycoprotein in multiple sclerosis.<sup>[12](https://pmc.ncbi.nlm.nih.gov/articles/PMC12499345/pdf/nihms-2114747.pdf)</sup> In cancer, tumors locally recruit regulatory T cells that protect them from immune clearance, which is why FOXP3 biology is also studied where the immune system should be activated rather than suppressed.<sup>[3](https://www.livescience.com/health/nobel-prize-in-medicine-goes-to-trio-for-their-work-on-immune-tolerance)</sup>

## References

1. [FOXP3 gene, MedlinePlus Genetics, National Library of Medicine](https://medlineplus.gov/download/genetics/gene/foxp3.pdf)
2. [Post-translational modification networks regulating FOXP3 function, PubMed](https://pubmed.ncbi.nlm.nih.gov/25047417/)
3. [Nobel Prize in medicine goes to trio for their work on immune tolerance, Live Science](https://www.livescience.com/health/nobel-prize-in-medicine-goes-to-trio-for-their-work-on-immune-tolerance)
4. [Metabolic regulation of forkhead box P3 alternative splicing isoforms and their impact on health and disease, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC10588449/)
5. [FoxP3 in Treg cell biology: a molecular and structural perspective, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC7008219/)
6. [Ramsdell, F. & Ziegler, S. F. (2014). FOXP3 and scurfy: how it all began. Nature Reviews Immunology 14, 343–349](https://www.nature.com/articles/nri3650)
7. [Molecular Mechanisms Controlling Foxp3 Expression in Health and Autoimmunity, Frontiers in Immunology](https://www.frontiersin.org/journals/immunology/articles/10.3389/fimmu.2019.03136/full)
8. [A splice of life: the discovery, function, and clinical implications of FOXP3 isoforms in autoimmune disease, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC12477319/)
9. [Mechanisms of human FoxP3+ Treg cell development and function in health and disease, Clinical and Experimental Immunology](https://academic.oup.com/cei/article/197/1/36/6402553)
10. [Introduction, PMC (IPEX syndrome article, 2025)](https://pmc.ncbi.nlm.nih.gov/articles/PMC12540517/pdf/10875_2025_Article_1934.pdf)
11. [IPEX and the role of Foxp3 in the development and function of human Tregs, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC1464917/)
12. [Cellular Therapies in Transplantation – Regulatory T Cell Therapies and Virus Specific Therapies, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC12499345/pdf/nihms-2114747.pdf)

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*Topic: Encyclopedia › Life and health › Biological foundations › RNA and gene regulation › Transcription and gene regulation › Transcription factor families and specific factors › Forkhead (FOX) transcription factors*

*Initially written Sep 17, 2026 · Reviewed: Oct 21, 2026 · Edited: Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*

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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license