Shares in video game retailer GameStop jumped more than 70% in Monday morning’s trade as investors jumped into the stock.

The surprise boom followed the online reappearance of Keith Gill, known as Roaring Kitty, who spearheaded the explosion in the struggling firms share price in January 2021, going against the bets of professional Wall Street firms.

Gill was one of the most high-profile of the group of independent investors who used online forums to swap investment tips. They drove a buying frenzy of shares in GameStop, which put pressure on hedge funds who had shorted the stock, expecting its price to fall. The story of the meme stock inspired the movie Dumb Money.

Gill had not posted on X (Twitter) since mid-2021, but on Sunday posted a graphic, following it up with two posts on Monday morning, alluding to his return.

History may be repeating itself, because the sudden resurgence in the share price has cost short sellers a loss of around $838 million after yesterday’s record-breaking rally, , according to data firm S3 Partners, cited by CNBC. The stock (NYSE:GME) closed up 74.40% at $30.45 after Roaring Kitty’s return.

Other meme stocks also surged on Monday: AMC (NYSE:AMC) jumped 15%, while Reddit (NYSE:RDDT) traded up 9%.