Twitter has threatened legal action against Meta over its new, text-based app called Threads, according to a letter obtained by Semafor.

In a Wednesday letter addressed to Meta CEO Mark Zuckerberg, Alex Spiro, an attorney representing Twitter, accused Meta of unlawfully using Twitter’s trade secrets and other intellectual property by hiring former Twitter employees to create a “copycat” app.

Since launching Threads Wednesday night, Meta’s new app has collected tens of millions of sign ups. The app, which was created by the company’s Instagram team, arrives at a time when many are looking for Twitter alternatives to escape Elon Musk’s raucous oversight of the platform since acquiring it last year for $44 billion.

Meta spokesperson Andy Stone responded to the report of Spiro’s letter on Threads Thursday afternoon, writing, “no one on the Threads engineering team is a former Twitter employee — that’s just not a thing.”

Treasury Secretary Janet Yellen plans to tell Chinese officials that Washington wants healthy economic competition but will defend trade curbs imposed by the U.S. on security grounds. She also will express concerns about Beijing’s export controls on metals used in semiconductors and solar panels, according to a senior Treasury official who briefed reporters after Yellen arrived in Beijing as part of efforts to revive strained relations. Treasury officials said earlier she wouldn’t meet with Chinese leader Xi Jinping.