Biotech · Nucleic acid therapeutics
Excepgen
A San Francisco biotech that started by engineering human cells to make hard to express proteins for drug discovery, and has since turned to a different problem: teaching cells to tolerate the mRNA and DNA medicines doctors want to give them.
Two starts, one company
Excepgen came out of Y Combinator's Winter 2019 batch with a pitch about protein expression. The company had engineered a human cell line to produce full length human proteins for drug discovery, a problem that had pushed most of the industry toward insect or E. coli cells and truncated protein fragments instead. “When human cells produce the wrong protein, or the protein in the wrong amount, this is where diseases usually come from. Our cells have had billions of years to specialize in fighting against this,” Barbara Mertins, the company's chief scientific officer and co-founder, told Fierce Biotech in 2019.
By the time the company re-emerged publicly in August 2023, after what its own release called a period of quiet advancement, the pitch had changed. The website no longer talks about protein expression for screening. It talks about vaccines.
The problem it is chasing now
Nucleic acid medicines, mRNA and DNA based drugs and vaccines, work by handing a cell a set of instructions. The trouble is that cells have spent billions of years learning to reject foreign genetic material. “Cells have evolved to reject foreign mRNA and DNA, which limits their potential when delivered as a vaccine or a therapeutic,” Folliard said in a 2023 interview with Oxonian Ventures, one of the company's investors. That rejection shows up clinically as reactogenicity, the fevers, chills and injection site reactions that limit how much of a vaccine a person can tolerate.
Excepgen's answer is a second piece of mRNA, delivered alongside the main therapeutic cargo, that encodes a Cardiovirus leader protein. The company calls the combination RNAx. The leader protein is known in virology for blocking nucleocytoplasmic transport, the traffic of molecules between a cell's nucleus and its cytoplasm, which is one of the ways cells sense and respond to foreign RNA. By dialing that traffic down, Excepgen says it can quiet the alarm response without silencing the therapeutic signal itself.
An Oxford biochemist and a co-founder
Thomas Folliard completed a DPhil in biochemistry at Oriel College, Oxford, splitting his time across the engineering, biochemistry and plant sciences departments. “I don’t see myself as an entrepreneur, I see myself as a scientist who discovered something that fitted best into starting a company,” he told Oxonian Ventures. Barbara Mertins is the company's other named co-founder and its chief scientific officer. Katherine Barcay, listed as chief operating officer on the company's 2025 publication and on a VitaDAO project page, rounds out the leadership named in public records.
The Fierce Biotech profile described a lean launch team of three, with Excepgen already working with three unnamed big pharma partners and naming one: AstraZeneca, whose vice president of discovery biology, Steve Rees, said the company was “constantly on the lookout for new methods” to express proteins pharma companies had struggled to make in house.
From stealth to a paper
On August 24, 2023, Excepgen announced it had closed a $4 million round, bringing total seed funding to $14 million, led by RA Capital Management and backed by Gravity Fund, Apollo Projects (the Sam Altman vehicle) and Czech investor ZAKA VC. The same year, the decentralized science funder VitaDAO committed $100,000 to an Excepgen project aimed at a universal pan flu vaccine that pairs a self amplifying RNA vaccine with RNAx to cut inflammatory side effects. VitaDAO's board called the project's data package strong and noted the company's funding and partnerships when it recommended the grant.
Almost two years later, on April 29, 2025, Excepgen scientists published the first peer reviewed data behind RNAx in NPJ Vaccines, describing experiments in human cells and mice.
What is proven, and what is still claimed
| Evidence | What the record shows | Source type |
|---|---|---|
| NPJ Vaccines paper | Published Apr 29, 2025 (with a publisher correction in Jun 2025). Co administering RNAx with a self amplifying RNA vaccine cargo reduced interferon production and serum biomarkers of reactogenicity while enhancing influenza hemagglutinin expression in human primary cells and mice, and preserved or increased antibody and neutralization titers. | Public record |
| Patent applications | The paper's competing interests statement lists four pending international applications naming Excepgen: WO 2021/055369, WO 2022/192694, WO 2022/197940 and WO 2023/183889, covering NCT inhibitor proteins for protein expression and vaccines. A related US filing, US20240141373A1, shares a 2019 priority date with the original protein expression platform. | Public record |
| AstraZeneca collaboration | Named in 2019 as an active, early stage collaboration to express hard to make proteins. No update, renewal or result has been reported since. | Independent, 2019 |
| Total seed funding | Company reports $14 million as of Aug 2023. A third party research database itemizes $150,000 (Mar 2019), $125,000 (Aug 2019), $9.0 million (Jul 2020) and $4.0 million (Aug 2023), which sums to about $13.3 million, close to but not matching the company's figure. | Differs slightly |
| SEC Form D filings | None found. A search of SEC EDGAR's company database and its full text search for “Excepgen” returned zero matching filings as of Sep 25, 2026, despite the company naming institutional investors publicly. | Not found |
| FDA, clinical trials, federal grants | No FDA 510(k) or PMA record, no registered clinical trial, and no NIH or NSF award was found under the company name. | Not found |
| Named clinic or pharma customers for RNAx | No named customer or licensing partner has been reported for the RNAx platform specifically. | Not found |
Read plainly: Excepgen has one peer reviewed paper and a family of pending patents behind its current platform, both public record. The 2019 pharma collaboration that once anchored its story has gone quiet in public reporting. And a company that says it raised $14 million from named institutional investors has no Form D on file with the SEC under its own name, a gap worth watching rather than assuming away.
What to watch
- Whether the pan flu vaccine program VitaDAO funded advances into further animal or human studies.
- Any update on the AstraZeneca collaboration first reported in 2019, or new named partners for RNAx.
- A Form D or other SEC filing that would confirm the terms of the $14 million the company says it has raised.
- Additional peer reviewed publications beyond the single 2025 NPJ Vaccines paper.
In their words
“When human cells produce the wrong protein, or the protein in the wrong amount, this is where diseases usually come from. Our cells have had billions of years to specialize in fighting against this.”
Barbara Mertins, chief scientific officer and co-founder, 2019 · Independent
“We were recently introduced to ExcepGen; they have an exciting new platform that can rapidly express proteins in mammalian cells with the potential to successfully express these larger proteins.”
Steve Rees, VP of discovery biology, AstraZeneca, 2019 · Independent
“Our mission at ExcepGen is to unlock nucleic acid medicines. This funding validates our aim to make RNA and DNA-based drugs a cornerstone of therapeutics.”
Thomas Folliard, CEO and co-founder, Aug 2023 release · Company release
“We are excited to support the potential of ExcepGen’s novel platform to enhance nucleic acid therapeutics across a broad range of indications.”
Cosmas Giallourakis, Venture Partner, RA Capital Management, Aug 2023 · Company release
“Engineering cellular response to mRNA vaccines and therapeutics represents a significant leap forward in the development of mRNA technology and, we believe, ultimately allows mRNA technology to reach its full potential.”
Seth DeGroot, partner, Gravity Fund, Aug 2023 · Company release
“I don’t see myself as an entrepreneur, I see myself as a scientist who discovered something that fitted best into starting a company.”
Thomas Folliard, Oxonian Ventures founder interview, 2023 · Interview
“We are gradually expanding our portfolio with other companies in this industry and, like other large investors from Silicon Valley, we see great potential in technologies that positively affect the development and production of vaccines and therapeutics.”
Jan Kasper, managing partner and co-founder, ZAKA VC, Sep 2023 · Investor
Related companies
Sources
- Public recordY Combinator company page, Excepgen
- Public recordNPJ Vaccines paper, Wojcechowskyj et al., with competing interests and patent list
- Public recordPubMed publisher correction record, PMID 40500262
- Public recordUS20240141373A1, Systems and methods for protein expression
- Public recordSEC EDGAR company search and full text search for Excepgen, zero results
- IndependentExcepGen emerges from Y Combinator, taking aim at better proteins for drug discovery
- IndependentExcepGen: Genetic Medicine Company Closes $14 Million In Total Seed Funding
- InterviewMeet Thomas Folliard, CEO and Founder, of ExcepGen
- InvestorZAKA VC expands its global biotechnology portfolio and invests in ExcepGen
- IndependentExcepGen Inc, RNA therapeutics for longevity, project and board evaluation
- IndependentExcepGen company profile
- IndependentAn mRNA that suppresses host innate signaling could augment self-amplifying RNA vaccines
- IndependentExcepgen company and funding research summary
- CompanyExcepGen Emerges From Stealth with New Funding for its Nucleic Acid Therapeutics Platform
- CompanyExcepGen homepage
- CompanyExcepGen team page
- CompanyExcepGen careers page
- CompanyExcepGen contact page, 780 Bryant St address
Profile researched and written by Healthcare Discovery. Last updated September 29, 2026.
