WASHINGTON (NewsNation Now) — Republican Senator Rand Paul again pressed Dr. Anthony Fauci about COVID-19 at a Senate hearing, this time over an unproven theory that the virus originated in a lab in Wuhan, China.

Paul began his line of questioning, which went longer than his allotted 5 minutes, with a probe into gain of function research. That’s when scientists artificially mutate known viruses. It can be done to study potential cures and viruses, but it could also lead to disaster if an experiment went wrong.

Paul said the U.S. has funded gain of function research in this country, and a North Carolina expert on coronaviruses, Dr. Raph Baric, had shared his results with another scientist at the Wuhan Virology Lab, Dr. Shi Zhengli, before the pandemic.

“Senator Paul, with all due respect, you are entirely and completely incorrect, that the [National Institutes of Health] has not ever and does not now fund gain of function research in the Wuhan Institute,” Fauci, director of the National Institute of Allergy and Infectious Diseases and chief medical adviser to President Biden, said. “Dr. Baric does not do gain of function research. And if it is, it’s according to the guidelines and it is being conducted in North Carolina, not China.”

Baric is a distinguished professor a the University of North Carolina, and runs a lab that “uses coronaviruses as models to study the genetics of RNA virus transcription, replication, persistence, pathogenesis, genetics and cross-species transmission,” according to the university’s website. Baric has been the subject of conspiracy theories since early in the pandemic.

An e-mail to Baric asking for a response to Paul’s allegations was not immediately returned.

Paul also asked why the U.S. previously funded the Wuhan lab, which some believe may be the origin of SARS-CoV-2, the virus that causes COVID-19. Fauci said it was in response to the original SARS outbreak in the 2000s. He said the U.S. government wanted more research on bat-transmitted diseases.

“I do not have any accounting of what the Chinese may have done and I’m fully in favor of any further investigation of what went on in China,” Fauci said. “However, I will repeat again, the NIH and NIAID categorically has not funded gain of function research to be conducted in the Wuhan Institute.”

After Paul’s time was up, Democratic Senator Tina Smith of Minnesota asked Fauci what the impact of “conspiracy theories peddled by Rand Paul and others” had on combatting vaccine hesitancy.

“They really are not helpful with what we’re trying to do. I think I can say that with some degree of confidence,” Fauci said.

Fauci and Paul have sparred at previous Senate hearings. Paul has been vocal about his opposition to federally imposed pandemic restrictions. He tweeted in April that Fauci was a “petty tyrant” for suggesting fully vaccinated Americans should avoid indoor dining.

The senator has also railed against guidance calling for vaccinated people to wear masks.

At a Senate hearing in September 2020, the Kentucky senator asked Fauci why he would praise New York Gov. Andrew Cuomo when the state had a high cumulative total of COVID-19 deaths.

Fauci said, “no, you’ve misconstrued that, Senator, and you’ve done that repetitively in the past.” He went on to explain that his praise was directed at the state’s job in quelling their initial outbreak, and by September their positivity rate was 1%.

The feud goes back further. At a May 2020 hearing, Paul pressed Fauci on when it would be safe for businesses and schools to re-open.

“I think we ought to have a little bit of humility in our belief that we know what’s best for the economy, and as much as I respect you, Dr. Fauci, I don’t think you’re the end-all,” Paul said.

Fauci responded by saying he “never made myself out to be the end-all” voice, and that other advisors were also weighing in.

Centers for Disease Control and Prevention Director Dr. Rochelle Walensky, Dr. Peter Marks, director of FDA’s Center for Biologics Evaluation and Research, and Dr. David Kessler, HHS chief science officer for COVID response, also testified at Tuesday’s hearing.