The upstart stock exchange, founded and backed by many of Wall Street’s biggest financial firms, has added Citigroup to its group of investors, it announced Thursday. The Members Exchange, also known as MEMX, is slated to launch Sept. 4 and aims to shake up the New York Stock Exchange-Nasdaq duopoly that has long dominated the U.S. equities market.

Citi’s investment comes in the wake of a $65 million funding round sealed by MEMX earlier this year. That round saw a wave of new investors—including JPMorgan Chase, Goldman Sachs, BlackRock, and Wells Fargo—join an initial group of seed backers that featured Morgan Stanley, Bank of America, Charles Schwab, Citadel Securities, and Fidelity Investments. MEMX had raised a total of $135 million in funding prior to Citi’s backing, according to PitchBook data.

“Although we recently closed our latest round of financing and are focused on launch, we didn’t want to pass up the opportunity to add another significant market participant to our list of supporters,” MEMX CEO Jonathan Kellner said in a statement. “We look forward to working with the Citi team and leveraging their market structure expertise.”

Kellner also announced that MEMX has received approval from the Financial Industry Regulatory Authority (FINRA) for its routing broker-dealer, MEMX Execution Services. The fledgling exchange received the Securities and Exchange Commission’s approval to operate in May.

Wall Street firms have flocked to MEMX amid heightened disquiet in recent years regarding the fees that the NYSE and Nasdaq charge broker-dealers. The exchange aims to offer a streamlined, more transparent platform that aims to “lower pricing on market data, connectivity, and transaction fees,” according to its website.

But breaking the NYSE and Nasdaq’s stranglehold has proven difficult in the past for upstart exchanges. The Investors Exchange (IEX)—which featured prominently in Michael Lewis’s 2014 book Flash Boys: A Wall Street Revolt—launched to much fanfare in 2016; today, IEX only garners around 2% of U.S. stock trading volume.

MEMX plans to begin platform testing on June 29, with member certification starting on July 13. The exchange is currently working to connect and onboard more than 50 market participants to its platform, Kellner said, with the goal of “launch[ing] with a truly diverse set of liquidity.”