# RNA-binding protein FUS
**RNA-binding protein FUS**, commonly called **FUS** (fused in sarcoma) and historically **TLS** (translocated in liposarcoma), is a 526-amino-acid, 75 kDa DNA/[RNA-binding protein](https://www.edgepedia.org/rna-binding-protein) of the heterogeneous nuclear ribonucleoprotein (hnRNP) complex that regulates transcription, RNA processing, RNA transport, and [DNA repair](https://www.edgepedia.org/dna-repair).[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC6514960/) • [2](https://www.ncbi.nlm.nih.gov/mesh/68034702) The protein is best known for two reasons: chromosomal translocations that fuse its N-terminal region to transcription factors produce oncogenes in myxoid liposarcoma and acute myeloid leukemia, and mutations in the FUS gene cause a familial form of amyotrophic lateral sclerosis (ALS).[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC6514960/) • [2](https://www.ncbi.nlm.nih.gov/mesh/68034702)
| Key fact | Detail |
|---|---|
| Protein size and class | 526 amino acids, 75 kDa; a multifunctional hnRNP that binds RNA, single-stranded DNA, and possibly double-stranded DNA[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC6514960/) |
| Family | FET family of RNA-binding proteins, together with EWSR1 (Ewing sarcoma protein) and TAF15[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC4565867/) |
| Cancer role | N-terminal FUS fusions to transcription factors form oncogenes in myxoid liposarcoma and acute myeloid leukemia; the gene was first identified as a fusion oncogene on chromosome 16 in liposarcoma[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC6514960/) • [2](https://www.ncbi.nlm.nih.gov/mesh/68034702) |
| Neurodegenerative role | FUS gene mutations cause ALS-FUS, one of the recognized genetic subtypes of ALS, a disease affecting about two per 100,000 people worldwide[4](https://www.sciencedirect.com/science/article/pii/S0301008216300740) |
| Pathology | Mutant FUS redistributes from the nucleus to the cytoplasm, aggregates, and joins stress granules; TDP-43 pathology is absent in ALS-FUS cases[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC6514960/) • [5](https://pmc.ncbi.nlm.nih.gov/articles/PMC5101148/) |
| Phase behavior | FUS undergoes liquid-liquid phase separation, scaffolding membraneless organelles; ALS mutations increase its tendency to form highly stable condensates[6](https://www.sciencedirect.com/science/article/pii/S0006899318302312) |
| Therapy | The antisense drug ulefnersen met its Phase 3 FUSION primary endpoint (p = 0.0005), what Ionis described as the first placebo-controlled clinical study targeting the genetic cause of FUS-ALS[7](https://ir.ionispharma.com/node/35116/pdf) |
## Structure and domains
FUS belongs to the FET/TET family, whose members share domains of similar structure and function.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC6514960/) Its domain map runs from an N-terminal glutamine-glycine-serine-tyrosine-rich (QGSY) transcriptional activation domain (amino acids 1–165), through three arginine-glycine-glycine repeat regions (RGG1–RGG3), an [RNA recognition motif](https://www.edgepedia.org/rna-recognition-motif) (amino acids 285–371), a Cys2–Cys2 zinc finger, a nuclear export signal, and a C-terminal proline-tyrosine nuclear localization signal (PY-NLS).[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC6514960/) • [6](https://www.sciencedirect.com/science/article/pii/S0006899318302312) Bioinformatic analysis predicts two prion-like domains, at amino acids 1–239 and 391–407.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC6514960/) In a prediction algorithm scoring 27,879 human proteins against a yeast prion domain, FUS ranked 15th overall for prion-like character and first among RNA-binding proteins.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC6514960/)
The RGG–zinc finger–RGG region is considered the major RNA-binding sequence, and RNA oligoribonucleotides bind FUS via the GGUG motif; the most important binding sequence overall is GUGGU, which is rich in 5′ untranslated regions, while mutant FUS preferentially binds 3′-UTR and intron sequences.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC6514960/)
## Normal functions
**RNA metabolism.** FUS binds thousands of cellular RNAs and participates in pre-mRNA splicing, mRNA stability, mRNA transport, and microRNA biogenesis; it co-purifies with the spliceosome and with Drosha microRNA processing complexes.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC4565867/) It binds the whole length of nascent RNA, implying close association with transcripts from the moment of production and a possible role in transcriptional elongation.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC4565867/) As a transporter, FUS moves mRNA between the cytoplasm and the nucleus, including transfer between neuronal dendrites and dendritic spines, which is described as essential for neuronal maturation, plasticity, and dendrite integrity.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC6514960/) In neurons, FUS also modulates the RNA metabolism of synaptic proteins such as GluA1 and SynGAP, affecting dendritic spine maturation and excitatory synaptic transmission.[8](https://academic.oup.com/brain/advance-article-pdf/doi/10.1093/brain/awaf448/65667450/awaf448.pdf)
**Transcription regulation.** FUS binds single-stranded DNA motifs in promoter regions transcribed by RNA polymerases II and III, accumulates near transcription start sites, and changes transcription levels; it also interacts with transcription factors including Spi-1/PU.1, NF-κB, and Runx2, and can recruit [RNA polymerase II](https://www.edgepedia.org/rna-polymerase-ii) and alter its phosphorylation status.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC6514960/) The N-terminal portion of the protein is a potent transcriptional activator, which is why fusion proteins containing it act as strong activators in cancer.[2](https://www.ncbi.nlm.nih.gov/mesh/68034702)
**DNA repair.** FUS is among the proteins recruited earliest to DNA damage sites, and its loss impairs ATM/γH2AX signaling; the mechanism depends on interactions with histone deacetylase 1 and poly-ADP ribose, a by-product of DNA damage.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC6514960/) FUS participates in homologous DNA pairing, promoting complementary single-stranded DNA annealing and D-loop formation, a principal step in double-strand break repair by recombination.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC6514960/) • [2](https://www.ncbi.nlm.nih.gov/mesh/68034702) Knockdown of FUS during cell growth leads to defects in DNA damage recovery.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC6514960/)
## Phase separation and stress granules
Many RNA-binding proteins, including FUS, contain low-complexity, intrinsically disordered domains that underpin reversible liquid-liquid phase separation (LLPS), allowing the proteins to serve as scaffolds for membraneless organelles such as nucleoli, stress granules, and neuronal transport granules.[6](https://www.sciencedirect.com/science/article/pii/S0006899318302312) FUS undergoes LLPS in vivo and in vitro at physiological concentrations, and phase separation at DNA damage sites locally concentrates FUS, possibly promoting repair.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC6514960/)
The nuclear import receptor Kapβ2 (transportin 1) both drives FUS nuclear import through its PY-NLS and acts as a chaperone that blocks and even dissolves FUS phase separation in vitro.[9](https://www.frontiersin.org/journals/cell-and-developmental-biology/articles/10.3389/fcell.2022.840256/full) All RGG repeats of FUS are methylated by the arginine methyltransferases PRMT1 or PRMT8, and this methylation is disrupted in patients, impairing FUS LLPS and its localization within stress granules.[9](https://www.frontiersin.org/journals/cell-and-developmental-biology/articles/10.3389/fcell.2022.840256/full) RNA abundance also tunes the phase behavior: high RNA concentration inhibits phase separation, while low RNA concentration promotes it.[10](https://onlinelibrary.wiley.com/doi/10.1002/med.21937)
## FUS in ALS and FTD
The first FUS mutation associated with ALS was discovered in 2009, and FUS-immunoreactive cytoplasmic inclusions were subsequently found in ALS-FUS patients.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC6514960/) ALS-FUS is one of at least five distinct pathological subtypes of the disease, alongside ALS-SOD1, ALS-C9orf72, ALS-TDP-43, and Guam-ALS.[4](https://www.sciencedirect.com/science/article/pii/S0301008216300740) A distinguishing feature is that TDP-43 pathology, a hallmark of most ALS cases, is consistently absent from ALS-FUS.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC5101148/) • [11](https://molecularneurodegeneration.biomedcentral.com/counter/pdf/10.1186/s13024-025-00851-y)
**Mislocalization mechanism.** Most familial ALS mutations alter the C-terminal nuclear localization signal, preventing FUS from binding its transport receptor and accumulating in the cytoplasm.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC6514960/) Excess cytoplasmic FUS joins stress granules, membraneless assemblies of mRNAs, translation initiation factors, and other RNA-binding proteins induced by stresses such as oxidative stress, mitochondrial dysfunction, and viral infection.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC6514960/) ALS-linked mutations in the intrinsically disordered region, such as G156E, promote aggregate formation by accelerating the conversion of liquid FUS droplets into fibrillar aggregates, and mutations in the C-terminal domain alter RNA binding and promote aggregated FUS-containing ribonucleoprotein complexes.[9](https://www.frontiersin.org/journals/cell-and-developmental-biology/articles/10.3389/fcell.2022.840256/full) Familial ALS mutations increase the propensity of FUS to form highly stable condensates, and stable fibrillar condensates sequester cargo and impair ribonucleoprotein granule function.[6](https://www.sciencedirect.com/science/article/pii/S0006899318302312)
**Disease mechanisms.** Two models of cytoplasmic aggregation are described: a gain-of-function model in which cytoplasmic FUS sequesters important regulators or triggers abnormal signaling, and a loss-of-function model in which aggregation depletes functional FUS, especially in dendritic spines, destabilizing mRNA and triggering inflammation; nuclear loss additionally affects transcription, alternative splicing, and DNA repair.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC6514960/) Recent evidence supports a toxic gain-of-function mechanism driven by cytoplasmic accumulation: heterozygous FUS knock-in mice develop motor neuron degeneration while haploinsufficient knockout mice do not, and overexpression of wild-type FUS alone can trigger motor neuron degeneration.[8](https://academic.oup.com/brain/advance-article-pdf/doi/10.1093/brain/awaf448/65667450/awaf448.pdf) FUS cytoplasmic aggregation may also spread across anatomical networks in a prion-like manner.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC6514960/)
Beyond ALS, FUS aggregation has been observed in frontotemporal lobar degeneration and the polyglutamine diseases, including [Huntington's disease](https://www.edgepedia.org/huntingtons-disease), spinocerebellar ataxia, and dentatorubral-pallidoluysian atrophy; in FTLD, FUS usually coaggregates with other FET proteins, whereas in ALS with FUS inclusions the FUS mutant dominates the inclusion composition.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC6514960/) FUS mutations have also been detected in patients with essential tremor and [Parkinson's disease](https://www.edgepedia.org/parkinsons-disease).[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC6514960/)
## Therapeutic targeting
The antisense oligonucleotide [ulefnersen](https://www.edgepedia.org/ulefnersen), developed by [Ionis Pharmaceuticals](https://www.edgepedia.org/ionis-pharmaceuticals) and licensed to Otsuka in 2024, is designed for FUS-ALS. In the Phase 3 FUSION study it met its primary endpoint, a joint rank analysis of time to death or permanent ventilation, time to rescue, and change in the ALS Functional Rating Scale-Revised score from baseline to Day 505 versus placebo (p = 0.0005); Ionis described it as the first placebo-controlled clinical study targeting the underlying genetic cause of FUS-ALS, and the company planned discussions with the FDA and other regulators on expedited submission pathways.[7](https://ir.ionispharma.com/node/35116/pdf)
## References
1. [Chen C, Ding X, Akram N, Xue S, Luo S-Z (2019). Fused in Sarcoma: Properties, Self-Assembly and Correlation with Neurodegenerative Diseases. Molecules 24:1622, PMC6514960.](https://pmc.ncbi.nlm.nih.gov/articles/PMC6514960/)
2. [RNA-Binding Protein FUS, MeSH, NCBI.](https://www.ncbi.nlm.nih.gov/mesh/68034702)
3. [The function of RNA-binding proteins at the synapse: implications for neurodegeneration, PMC4565867.](https://pmc.ncbi.nlm.nih.gov/articles/PMC4565867/)
4. [TDP-43/FUS in motor neuron disease: Complexity and challenges. Progress in Neurobiology 145–146:78-97 (2016).](https://www.sciencedirect.com/science/article/pii/S0301008216300740)
5. [TDP-43/FUS in Motor Neuron Disease: Complexity and Challenges, PMC5101148.](https://pmc.ncbi.nlm.nih.gov/articles/PMC5101148/)
6. [The physiological and pathological biophysics of phase separation and gelation of RNA binding proteins in ALS and FTLD. Brain Research 1693:11-23 (2018).](https://www.sciencedirect.com/science/article/pii/S0006899318302312)
7. [Ionis announces positive topline results from Phase 3 FUSION study of ulefnersen for FUS-ALS.](https://ir.ionispharma.com/node/35116/pdf)
8. [FUS and TDP-43 in neurodegeneration across the ALS-FTLD spectrum. Brain (2025).](https://academic.oup.com/brain/advance-article-pdf/doi/10.1093/brain/awaf448/65667450/awaf448.pdf)
9. [Emerging Roles for Phase Separation of RNA-Binding Proteins in Cellular Pathology of ALS. Frontiers in Cell and Developmental Biology (2022).](https://www.frontiersin.org/journals/cell-and-developmental-biology/articles/10.3389/fcell.2022.840256/full)
10. [Molecular mechanisms of ALS as broad therapeutic targets for gene therapy applications utilizing AAV vectors. Medicinal Research Reviews (2023).](https://onlinelibrary.wiley.com/doi/10.1002/med.21937)
11. [RNA-binding proteins in ALS and FTD: from pathogenic mechanisms to therapeutic insights. Molecular Neurodegeneration (2025).](https://molecularneurodegeneration.biomedcentral.com/counter/pdf/10.1186/s13024-025-00851-y)
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*Topic: Encyclopedia › Life and health › Biological foundations › RNA and gene regulation › RNA processing, modification, and translation › RNA-binding proteins and helicases › Heterogeneous nuclear ribonucleoproteins (hnRNP)*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 21, 2026 · Edited: Sep 30, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license