# C-Raf

**C-Raf** (also called Raf-1 or CRAF, encoded by the RAF1 gene in humans) is a serine/threonine protein kinase that acts as a direct effector of the Ras GTPases and initiates the MAP kinase cascade by phosphorylating MEK1 and MEK2, which in turn activate ERK1 and ERK2.<sup>[1](https://www.ncbi.nlm.nih.gov/gene/005894)</sup> It was the first RAF kinase discovered in mammals and remains a central node in signaling pathways controlling cell division, apoptosis, differentiation, and migration.<sup>[1](https://www.ncbi.nlm.nih.gov/gene/005894)</sup><sup> • </sup><sup>[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC9101324/)</sup>

| Key fact | Detail |
|---|---|
| Gene and protein | RAF1 (Raf-1 proto-oncogene, serine/threonine kinase) at chromosome 3p25.2, 23 exons; aliases CRAF, Raf-1, c-Raf, NS5, CMD1NN<sup>[1](https://www.ncbi.nlm.nih.gov/gene/005894)</sup> |
| Pathway position | MAP kinase kinase kinase (MAP3K) downstream of Ras; phosphorylates MEK1/2, which activate ERK1/2<sup>[1](https://www.ncbi.nlm.nih.gov/gene/005894)</sup> |
| Size | 73 kDa, intermediate between A-Raf (68 kDa) and B-Raf (75 to 100 kDa)<sup>[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC9101324/)</sup> |
| Discovery | Cellular counterparts C-Raf-1 and C-Raf-2 cloned and sequenced in 1985; C-Raf-2 proved to be a pseudogene<sup>[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC9101324/)</sup> |
| Disease links | Mutations associated with Noonan syndrome 5 and LEOPARD syndrome 2; hyperactivation drives tumor progression and drug resistance<sup>[1](https://www.ncbi.nlm.nih.gov/gene/005894)</sup><sup> • </sup><sup>[3](https://pubmed.ncbi.nlm.nih.gov/38195917/)</sup> |
| Drug relevance | Target of sorafenib, the first oral multikinase inhibitor against Raf, FDA-approved for advanced renal cell carcinoma in December 2005<sup>[4](https://www.nature.com/articles/nrd2130)</sup> |

## Discovery and gene

The RAF name derives from the transforming gene of a murine retrovirus, v-raf (rapidly accelerated fibrosarcoma). The first cellular counterparts, C-Raf-1 and C-Raf-2, were cloned and sequenced in 1985; C-Raf-2 was soon recognized as a pseudogene, so the C-Raf-1 gene product was designated C-Raf.<sup>[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC9101324/)</sup> The human gene sits at chromosomal band 3p25 and contains 23 exons.<sup>[1](https://www.ncbi.nlm.nih.gov/gene/005894)</sup><sup> • </sup><sup>[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC9101324/)</sup> Two later mammalian family members, A-Raf and B-Raf, were identified in vertebrates after C-Raf.<sup>[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC9101324/)</sup>

Expression is broad: the NCBI gene record lists ubiquitous expression, with the highest levels in bone marrow (RPKM 34.4) and heart (RPKM 32.5).<sup>[1](https://www.ncbi.nlm.nih.gov/gene/005894)</sup>

## Structure and regulation

C-Raf shares with the other RAF kinases a three-conserved-region architecture: an N-terminal regulatory region (CR1 containing the Ras-binding domain and the CRD, CR2, and a C-terminal CR3 kinase domain).<sup>[5](https://www.sciencedirect.com/science/article/pii/S0014579305003522)</sup><sup> • </sup><sup>[6](https://diposit.ub.edu/bitstreams/c495a943-09e8-4c05-ad8f-d08b99bf2802/download)</sup> In the inactive state the N-terminal regulatory domain suppresses the kinase domain, and 14-3-3 dimers bind phosphorylation sites at both the N- and C-termini to stabilize this autoinhibited state.<sup>[5](https://www.sciencedirect.com/science/article/pii/S0014579305003522)</sup><sup> • </sup><sup>[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC3976976/)</sup>

**Activation requires dimerization.** Under normal signaling, active Ras-GTP recruits RAF proteins to the plasma membrane, 14-3-3 is released from the N-terminal site, and activating phosphorylations occur in the kinase domain.<sup>[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC3976976/)</sup> Catalytic activity depends on an allosteric mechanism driven by side-to-side kinase domain dimerization, in which one protomer in the active conformation, marked by formation of a hydrophobic spine, allosterically activates its partner.<sup>[8](https://www.nature.com/articles/nrm3979.pdf)</sup><sup> • </sup><sup>[9](https://hdl.handle.net/10197/5060)</sup> The first hint of this came in 1996, when forced homodimerization of C-Raf with artificial constructs was shown to increase its enzymatic activity.<sup>[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC3976976/)</sup> In 2024, cryo-EM structures of dimerized CRAF kinase domains bound to a 14-3-3 dimer, including complexes containing MEK, showed an architecture highly similar to the corresponding BRAF dimers, indicating a conserved dimeric active state.<sup>[10](https://pmc.ncbi.nlm.nih.gov/articles/PMC11346419/)</sup>

The HSP90/CDC37 chaperone complex stabilizes CRAF protein levels and serves as a platform for RAF dephosphorylation by the PP5 phosphatase.<sup>[10](https://pmc.ncbi.nlm.nih.gov/articles/PMC11346419/)</sup> Phosphorylation of Ser-621 in the C terminus, the C-terminal 14-3-3 binding site, is required for dimerization-driven activation, and blocking it with an AMPK inhibitor suppresses RAF inhibitor-driven ERK activation in Ras-mutated cells.<sup>[11](https://www.jbc.org/article/S0021925820309017/pdf)</sup>

## Function in signaling and kinase-independent roles

Once activated, C-Raf phosphorylates MEK1 and MEK2, the only known protein substrates of RAF kinases, which phosphorylate ERK1 and ERK2; activated ERKs control gene expression involved in the cell division cycle, apoptosis, differentiation, and migration.<sup>[1](https://www.ncbi.nlm.nih.gov/gene/005894)</sup><sup> • </sup><sup>[12](https://pmc.ncbi.nlm.nih.gov/articles/PMC10396080/)</sup>

**C-Raf also acts without its kinase.** Work from the laboratory of [Mariano Barbacid](https://www.edgepedia.org/mariano-barbacid) showed that inactivating RAF1 in mouse models of sporadic KRAS- or KRAS/p53-induced lung cancer caused tumor regression without major toxicities, probably because ERK signaling was not affected by the deletion; a kinase-dead RAF1 mutant rescued the phenotype.<sup>[13](https://www.cancer.gov/research/key-initiatives/ras/news-events/dialogue-blog/2018/baccarini-rok)</sup> This kinase-independent role in regulating ERK-independent signaling suggests that targeting RAF1 in tumors should aim at reducing its expression rather than blocking its catalytic activity.<sup>[13](https://www.cancer.gov/research/key-initiatives/ras/news-events/dialogue-blog/2018/baccarini-rok)</sup> In RAS-driven epidermal tumorigenesis, RAF1 deficiency prevents tumor development through ROKα-induced keratinocyte differentiation, a phenotype reversible by topical ROK inhibitor treatment.<sup>[13](https://www.cancer.gov/research/key-initiatives/ras/news-events/dialogue-blog/2018/baccarini-rok)</sup>

## C-Raf in cancer

Hyperactivation of RAF proteins, particularly BRAF and CRAF, drives tumor progression and drug resistance in many cancer types; although BRAF is the more studied family member because of its high mutation incidence in melanoma, the role of CRAF in tumorigenesis and drug resistance is increasingly clinically relevant.<sup>[3](https://pubmed.ncbi.nlm.nih.gov/38195917/)</sup> RAF1 mutations can be categorized by their effect on kinase activity, and precision therapies for CRAF-dependent tumors are an active area of preclinical and clinical work.<sup>[3](https://pubmed.ncbi.nlm.nih.gov/38195917/)</sup> A 2026 [PLOS One](https://www.edgepedia.org/plos-one) study in KRAS-driven lung adenocarcinoma models found RAF1 a valid standalone therapeutic target, with no added efficacy from co-targeting ARAF, EGFR, or [DDR1](https://www.edgepedia.org/ddr1).<sup>[14](https://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0341778)</sup>

The first proof that the raf1 gene is a valid anticancer target came in 1989, when antisense knockdown inhibited human lung, breast, and ovarian tumor xenograft growth in mice; this led to sorafenib (BAY 43-9006), discovered through high-throughput screening begun in 1995 and FDA-approved for advanced renal cell carcinoma in December 2005.<sup>[4](https://www.nature.com/articles/nrd2130)</sup> More recently, the type II RAF inhibitor tovorafenib showed single-agent clinical activity with a manageable safety profile in a phase II FIRELIGHT-1 substudy of patients with solid tumors carrying BRAF fusions or CRAF/RAF1 fusions or amplification.<sup>[15](https://ascopubs.org/doi/10.1200/PO-25-01187)</sup>

## RAF inhibitors and paradoxical activation

ATP-competitive RAF kinase inhibitors block BRAF-mutant cells but can accelerate MAPK signaling in cells with wild-type BRAF and active RAS, a phenomenon called paradoxical activation.<sup>[16](https://pmc.ncbi.nlm.nih.gov/articles/PMC4134487/)</sup><sup> • </sup><sup>[17](https://www.nature.com/articles/nature08902)</sup> The accepted model holds that the effect is mediated by RAF dimers in which one protomer is occupied by the inhibitor and the other is not: the drug-bound protomer adopts an active conformation and allosterically transactivates the free partner, while at higher inhibitor concentrations both protomers are occupied and kinase activity falls.<sup>[16](https://pmc.ncbi.nlm.nih.gov/articles/PMC4134487/)</sup><sup> • </sup><sup>[17](https://www.nature.com/articles/nature08902)</sup> Because P-loop autophosphorylation is intrinsically linked to RAF catalytic activity, this mechanism predicts that all catalytic RAF inhibitors show some paradoxical activation in RAS-mutated, BRAF wild-type cells.<sup>[16](https://pmc.ncbi.nlm.nih.gov/articles/PMC4134487/)</sup> Inhibitors can also promote C-Raf dimerization with the pseudokinase KSR1 (kinase suppressor of Ras 1) rather than with another RAF.<sup>[18](https://hdl.handle.net/10197/9157)</sup>

Clinically, this paradox is the mechanistic basis for cutaneous squamous-cell carcinomas, gastrointestinal polyps, and other RAS-mutant malignancies arising in patients receiving BRAF inhibitor therapy; adding a MEK inhibitor both increases efficacy against BRAF-mutant melanoma and reduces the emergence of these RAS-mutant tumors.<sup>[19](https://www.annualreviews.org/doi/pdf/10.1146/annurev-med-090514-030732)</sup> Strategies to blunt the effect include pan-RAF inhibitors that inhibit ARAF, BRAF, and CRAF together, and "paradox breaker" compounds such as LY3009120, TAK-632, and PLX7904, which induce no or minimal paradoxical activation on BRAF-wild-type backgrounds.<sup>[20](https://www.nature.com/articles/s41598-018-37213-2)</sup><sup> • </sup><sup>[21](https://www.nature.com/articles/s41467-025-65376-w.pdf)</sup> Even selective CRAF inhibition is not expected to be spared: modeling results suggest selective CRAF inhibitors would elicit the same transactivation liability seen with BRAF-selective and pan-RAF inhibitors.<sup>[22](https://pubs.acs.org/doi/10.1021/jacs.0c11958)</sup> Paradoxical activation can also occur independently of classical RAS proteins, and pan-RAF inhibition combined with KRAS<sup>G12C</sup> inhibition produced unexpected ERK activation in KRAS<sup>G12C</sup>-dependent lung cancer cell lines.<sup>[23](https://pmc.ncbi.nlm.nih.gov/articles/PMC8812530/)</sup>

## References

1. [RAF1 Raf-1 proto-oncogene, serine/threonine kinase [Homo sapiens], NCBI Gene](https://www.ncbi.nlm.nih.gov/gene/005894)
2. [Discovery of Raf Family Is a Milestone in Deciphering the Ras-Mediated Intracellular Signaling Pathway, Int. J. Mol. Sci. (2022)](https://pmc.ncbi.nlm.nih.gov/articles/PMC9101324/)
3. [The role of CRAF in cancer progression: from molecular mechanisms to precision therapies, PubMed (2024)](https://pubmed.ncbi.nlm.nih.gov/38195917/)
4. [Discovery and development of sorafenib: a multikinase inhibitor for treating cancer, Nature Reviews Drug Discovery 5, 835-844 (2006)](https://www.nature.com/articles/nrd2130)
5. [Second nature: Biological functions of the Raf-1 kinase, FEBS Letters (2005)](https://www.sciencedirect.com/science/article/pii/S0014579305003522)
6. [The Mitogen-Activated Protein (MAP) kinase signalling pathway (RAF/MEK/ERK), University of Barcelona thesis chapter](https://diposit.ub.edu/bitstreams/c495a943-09e8-4c05-ad8f-d08b99bf2802/download)
7. [The importance of Raf dimerization in cell signaling, PMC (2014)](https://pmc.ncbi.nlm.nih.gov/articles/PMC3976976/)
8. [Regulation of RAF protein kinases in ERK signalling, Nature Reviews Molecular Cell Biology (2014)](https://www.nature.com/articles/nrm3979.pdf)
9. [Mode and function of Raf dimerization, Trinity College Dublin](https://hdl.handle.net/10197/5060)
10. [Regulation of RAF family kinases: new insights from recent structural and biochemical studies, Biochem J (2024)](https://pmc.ncbi.nlm.nih.gov/articles/PMC11346419/)
11. [The AMPK inhibitor overcomes the paradoxical effect of RAF inhibitors through blocking phospho-Ser-621 in the C terminus of CRAF, J Biol Chem (2020)](https://www.jbc.org/article/S0021925820309017/pdf)
12. [Mechanism and inhibition of BRAF kinase, PMC (2023)](https://pmc.ncbi.nlm.nih.gov/articles/PMC10396080/)
13. [Don't Miss the Connection: The Role of RAF1 and ROKα in RAS-Driven Cancers, NCI RAS Dialogue Blog (2018)](https://www.cancer.gov/research/key-initiatives/ras/news-events/dialogue-blog/2018/baccarini-rok)
14. [RAF1 as a standalone therapeutic target in KRAS-driven lung adenocarcinoma, PLOS One (2026)](https://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0341778)
15. [Type II RAF Inhibitor Tovorafenib in Recurrent/Refractory Melanoma or Other Solid Tumors With BRAF Fusions or CRAF/RAF1 Fusions or Amplification: Phase II FIRELIGHT-1 Substudy, JCO Precision Oncology](https://ascopubs.org/doi/10.1200/PO-25-01187)
16. [Mechanism and consequences of RAF kinase activation by small-molecule inhibitors, PMC (2014)](https://pmc.ncbi.nlm.nih.gov/articles/PMC4134487/)
17. [RAF inhibitors transactivate RAF dimers and ERK signalling in cells with wild-type BRAF, Nature (2010)](https://www.nature.com/articles/nature08902)
18. [RAF inhibitors and C-RAF:KSR1 dimerization, University College Dublin](https://hdl.handle.net/10197/9157)
19. [B-Raf Inhibition in the Clinic: Present and Future, Annual Review of Medicine](https://www.annualreviews.org/doi/pdf/10.1146/annurev-med-090514-030732)
20. [Development of Highly Sensitive Biosensors of RAF Dimerization in Cells, Scientific Reports (2019)](https://www.nature.com/articles/s41598-018-37213-2)
21. [RAF inhibitors activate the integrated stress response by direct activation of GCN2, Nature Communications (2025)](https://www.nature.com/articles/s41467-025-65376-w.pdf)
22. [Selective CRAF Inhibition Elicits Transactivation, Journal of the American Chemical Society (2021)](https://pubs.acs.org/doi/10.1021/jacs.0c11958)
23. [Classical RAS proteins are not essential for paradoxical ERK activation induced by RAF inhibitors, PMC (2022)](https://pmc.ncbi.nlm.nih.gov/articles/PMC8812530/)

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*Topic: Encyclopedia › Life and health › Biological foundations › Biochemistry and metabolism › Protein families and complexes › Kinase and phosphatase families › Protein kinase families › Protein tyrosine kinases › Csk, tyrosine kinase-like, and dual-specificity kinases*

*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