# Transcription factor
A transcription factor (TF) is a protein that binds to specific DNA sequences in enhancer or promoter regions and activates or represses the transcription of a particular gene.[1](https://www.ncbi.nlm.nih.gov/books/NBK10023/) By attaching themselves to DNA close to a gene as well as to proteins associated with DNA, transcription factors play a role in determining whether and in what manner a cell's genes are activated or silenced, influencing RNA production and, further along, protein synthesis and the varied functions those proteins carry out.[2](https://www.cancer.gov/publications/dictionaries/cancer-terms/def/transcription-factor) In current usage the term is reserved for proteins that both bind DNA in a sequence-specific manner and regulate transcription; historically it was applied more loosely to any protein involved in transcription.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC12908702/)
| Key fact | Detail |
|---|---|
| Definition | A protein that binds DNA sequence-specifically and regulates transcription, activating or repressing particular genes[1](https://www.ncbi.nlm.nih.gov/books/NBK10023/) • [3](https://pmc.ncbi.nlm.nih.gov/articles/PMC12908702/) |
| Scale in humans | 1,558 human TF genes classified, over 2,900 when counting isoforms from alternative splicing or protein processing[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC3531165/) |
| Largest superclass | Zinc-coordinating DNA-binding domains hold 53% of human TF genes, followed by helix-turn-helix (26%) and basic domain factors (11%)[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC3531165/) |
| Binding sites | Enhancers and promoters; first enhancer evidence came in 1980 from simian virus 40 and the sea urchin genome[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC8558291/) |
| Pioneer factors | A TF subclass that can bind nucleosome-embedded motifs and initiate opening of silent chromatin regions[6](https://pubmed.ncbi.nlm.nih.gov/37940484/) |
| Functional reach | On average 14.7% of genes bound by a factor changed expression after that factor was knocked down, so most binding events do not produce measurable expression changes[7](https://arxiv.org/abs/1310.5166) |
| Therapeutics | HIF-2 inhibitors showed effectiveness for several types of kidney cancer in clinical trials, a rare case of drugs targeting a transcription factor directly[8](https://time.com/6241441/kidney-cancer-clinical-trials/) |
## How transcription factors control genes
Most transcription factors bind to specific DNA sequences.[1](https://www.ncbi.nlm.nih.gov/books/NBK10023/) These sequences sit in promoters near the transcription start site and in enhancers, regulatory elements that can lie far from the gene they control. The first evidence for enhancers arose in 1980, when short DNA sequences were discovered within simian virus 40 and the sea urchin genome that were remote from a gene promoter yet stimulated gene expression by an unknown mechanism.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC8558291/) It was not long before enhancers turned up in immunoglobulin genes and other cellular genes, where they proved central to determining which genes a given cell type expresses; moreover, how many enhancers a eukaryotic genome contains tracks with how complex the organism is, and a typical mammalian gene is probably regulated by multiple enhancers that fine-tune its expression across developmental stages, cell types, and signaling inputs.[9](https://pubmed.ncbi.nlm.nih.gov/25719310/)
**Activation and repression.** A bound factor can recruit the transcription machinery, including proteins involved in binding [RNA polymerase](https://www.edgepedia.org/rna-polymerase) such as TFIIB or TFIIE, through its trans-activating domain, or it can repress transcription.[1](https://www.ncbi.nlm.nih.gov/books/NBK10023/) Factors frequently act in combination: cooperative binding of transcription factors to cis-regulatory regions is essential for the precision of gene expression in development and other processes.[10](https://arxiv.org/abs/0901.2905) Because different cell types express different sets of factors, the same gene can be read differently in each cell, which is how a single genome supports many cell types.
## Families and classification
Transcription factors are grouped into families based on similarities in structure.[1](https://www.ncbi.nlm.nih.gov/books/NBK10023/) Members of the same family have DNA-binding sites built on a shared structural framework, and small variations in the amino acids making up the site can change which DNA sequence the factor recognizes.[1](https://www.ncbi.nlm.nih.gov/books/NBK10023/) The TFClass database classifies mammalian DNA-binding transcription factors by their DNA-binding domains in a hierarchical schema of superclass, class, family, subfamily, genus (the TF gene), and factor species (the polypeptide); nine superclasses comprising 40 classes and 111 families cover 1,558 human TF genes.[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC3531165/) • [11](https://academic.oup.com/nar/article/41/D1/D165/1061041)
The largest superclass by gene count is the zinc-coordinating DNA-binding domains, to which 53% of all human TF genes belong, followed by helix-turn-helix factors at 26% and basic domain factors at 11%.[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC3531165/) [Homeodomain](https://www.edgepedia.org/homeodomain) factors, a helix-turn-helix group important in specifying anterior-posterior axes during development, are highly enriched for tissue-specific expression, with 133 of 162 (82%) showing it, consistent with roles in early embryonic cell fate specification and maintenance of differentiation in adult tissues.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC12908702/) • [1](https://www.ncbi.nlm.nih.gov/books/NBK10023/)
## Chromatin and pioneer factors
In eukaryotes, DNA is packaged in nucleosomes, and physical access to DNA is a dynamic property of chromatin that plays an essential role in establishing and maintaining cellular identity.[12](https://www.nature.com/articles/s41576-018-0089-8) Nucleosomes envelope nearly all of the genome, and transcription factors are believed to displace them at sites controlling gene expression; research on TF binding to naked versus nucleosomal DNA shows marked contrasts and a varied landscape of interactions between transcription factors and the nucleosome.[13](https://www.nature.com/articles/s41586-018-0549-5)
**Pioneer factors.** Pioneer factors are a subclass of transcription factors that can bind and initiate opening of silent chromatin regions.[6](https://pubmed.ncbi.nlm.nih.gov/37940484/) They recognize nucleosome-embedded motifs, invade compact chromatin, and generate open chromatin regions, and many possess structural features in their DNA-binding domains that accommodate nucleosome substrates.[14](https://pmc.ncbi.nlm.nih.gov/articles/PMC11444900/) After binding, pioneer factors regulate lineage-specific genes and enhancers, activate the zygotic genome after fertilization, guide cell fate transitions during development, and promote various forms of human cancers.[6](https://pubmed.ncbi.nlm.nih.gov/37940484/) [Chromosome](https://www.edgepedia.org/chromosome) translocations can form oncogenic fusion pioneer factors, such as PAX3-FOXO1 in rhabdomyosarcoma, which alter enhancer usage and the gene expression profile.[15](https://pmc.ncbi.nlm.nih.gov/articles/PMC10873006/)
## Reading the genome is not a simple code
How transcription factors recognize their cognate binding sites involves several features beyond the nucleotide sequence: the three-dimensional structure and flexibility of the factor and its binding site, binding in the presence of cofactors, cooperative binding between factors, chromatin accessibility and nucleosome occupancy, indirect cooperativity via competition with nucleosomes, pioneer activity, and [DNA methylation](https://www.edgepedia.org/dna-methylation).[16](https://pmc.ncbi.nlm.nih.gov/articles/PMC4149858/) More than 1,600 entries of protein-DNA structures have been deposited in the [Protein Data Bank](https://www.edgepedia.org/protein-data-bank), including structures solved by nuclear magnetic resonance spectroscopy.[16](https://pmc.ncbi.nlm.nih.gov/articles/PMC4149858/)
Binding alone does not equal regulation. In an analysis intersecting gene expression data with TF binding data within 10 kb of transcription start sites, on average 14.7% of genes bound by a factor were differentially expressed following knockdown of that factor, suggesting that most interactions between TF and chromatin do not result in measurable changes in expression of putative target genes.[7](https://arxiv.org/abs/1310.5166) Genome-wide binding experiments similarly show that the number of genes bound by a TF far exceeds the number of possible direct target genes.[17](https://arxiv.org/abs/1512.05573)
## Transcription factors in disease and drug development
Mutations and other changes in transcription factor genes can produce abnormal proteins that are too active or present in higher-than-normal amounts in some cancer cells, which may drive cancer growth.[2](https://www.cancer.gov/publications/dictionaries/cancer-terms/def/transcription-factor) Stress signals from the cell microenvironment and internal signals such as genetic mutations, gene fusions, and aberrant post-translational modification can lead to persistent activation or inactivation of factors, contributing to disease development.[18](https://pmc.ncbi.nlm.nih.gov/articles/PMC12249972/)
**Drugging TFs.** Transcription factors have historically been considered among the most challenging targets in drug discovery.[19](https://www.nature.com/articles/s41401-026-01877-8) HIF-2, a transcription factor that regulates a gene affecting the growth of kidney cancer tumors, was long considered undruggable because typical drugs could not bind its unusual structure; researchers at UT Southwestern identified a cavity in the HIF-2 protein that could be exploited with specially designed drugs, and clinical trials demonstrated that these HIF-2 inhibitors are effective for several types of kidney cancer.[8](https://time.com/6241441/kidney-cancer-clinical-trials/) Newer approaches include direct inhibitors, molecular glues, and PROTACs (proteolysis-targeting chimeras), which have shown success in preclinical and clinical studies.[18](https://pmc.ncbi.nlm.nih.gov/articles/PMC12249972/) Research has also expanded to target cofactors and upstream signaling pathways rather than the DNA-binding protein itself, and to RNA-based strategies such as antisense oligonucleotides, small interfering RNAs, and transcription factor decoy oligodeoxynucleotides.[20](https://pmc.ncbi.nlm.nih.gov/articles/PMC11970766/) A limitation of TF drug targeting is the lack of specificity in the cell-type expression of the gene targets and signaling pathways these factors regulate.[20](https://pmc.ncbi.nlm.nih.gov/articles/PMC11970766/)
**Artificial transcription factors.** Artificial transcription factors are synthetic molecular tools designed to regulate the expression of disease-associated genes, with applications in cell reprogramming, genetic screens, and disease treatment. Most engineering efforts to date have used microbial and viral protein parts, and potential immunogenicity, large size, inefficient delivery, and off-target effects remain obstacles to their development as therapeutics.[21](https://pmc.ncbi.nlm.nih.gov/articles/PMC11638881/pdf/main.pdf)
## Measuring transcription factor binding
Genome-wide TF binding is mapped mainly by chromatin immunoprecipitation followed by sequencing (ChIP-seq) and, more recently, by CUT&RUN (Cleavage Under Targets and Release Using Nuclease) and CUT&Tag.[22](https://academic.oup.com/nar/article/52/7/e40/7631395) The ENCODE consortium sets standards for TF ChIP-seq experiments, requiring two or more biological replicates, characterized antibodies, a matching input control, and 20 million usable fragments per replicate, with replicate concordance measured by the Irreproducible Discovery Rate (IDR).[23](https://www.encodeproject.org/chip-seq/transcription_factor/) [Chromatin](https://www.edgepedia.org/chromatin) accessibility assays such as ATAC-seq and DNase-seq provide a complementary view, since the organization of accessible chromatin across the genome reflects a network of interactions through which enhancers, promoters, insulators, and chromatin-binding factors cooperatively regulate gene expression.[12](https://www.nature.com/articles/s41576-018-0089-8)
## References
1. [Differential Gene Transcription, Developmental Biology, NCBI Bookshelf](https://www.ncbi.nlm.nih.gov/books/NBK10023/)
2. [Definition of transcription factor, NCI Dictionary of Cancer Terms](https://www.cancer.gov/publications/dictionaries/cancer-terms/def/transcription-factor)
3. [The Human Transcription Factors, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC12908702/)
4. [TFClass: an expandable hierarchical classification of human transcription factors, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC3531165/)
5. [Transcriptional enhancers and their communication with gene promoters, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC8558291/)
6. [Pioneer factors: roles and their regulation in development, PubMed](https://pubmed.ncbi.nlm.nih.gov/37940484/)
7. [The Functional Consequences of Variation in Transcription Factor Binding, arXiv](https://arxiv.org/abs/1310.5166)
8. [How to Participate in Clinical Trials for Kidney Cancer, TIME](https://time.com/6241441/kidney-cancer-clinical-trials/)
9. [Enhancers, enhancers - from their discovery to today's universe of transcription enhancers, PubMed](https://pubmed.ncbi.nlm.nih.gov/25719310/)
10. [Nucleosome-mediated cooperativity between transcription factors, arXiv](https://arxiv.org/abs/0901.2905)
11. [TFClass: a classification of human transcription factors and their rodent orthologs, Nucleic Acids Research](https://academic.oup.com/nar/article/41/D1/D165/1061041)
12. [Chromatin accessibility and the regulatory epigenome, Nature Reviews Genetics](https://www.nature.com/articles/s41576-018-0089-8)
13. [The interaction landscape between transcription factors and the nucleosome, Nature](https://www.nature.com/articles/s41586-018-0549-5)
14. [Pioneer Factors: Nature or Nurture?, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC11444900/)
15. [Pioneer Factors: Roles and their regulation in development, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC10873006/)
16. [Absence of a simple code: how transcription factors read the genome, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC4149858/)
17. [Functional transcription factor target discovery via compendia of binding and expression profiles, arXiv](https://arxiv.org/abs/1512.05573)
18. [Transcription Factors and Methods for the Pharmacological Correction of Their Activity, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC12249972/)
19. [Transcription factor targeting strategies in cancer, Acta Pharmacologica Sinica](https://www.nature.com/articles/s41401-026-01877-8)
20. [Transcription factors in the development and treatment of immune disorders, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC11970766/)
21. [Development of artificial transcription factors and their applications, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC11638881/pdf/main.pdf)
22. [Exhaustive identification of genome-wide binding events of transcriptional regulators, Nucleic Acids Research](https://academic.oup.com/nar/article/52/7/e40/7631395)
23. [Transcription Factor ChIP-seq Data Standards and Processing Pipeline, ENCODE](https://www.encodeproject.org/chip-seq/transcription_factor/)
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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 › Transcription factors: overview and general treatment*
*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