PlayerUnknown’s Battlegrounds soared in popularity last year, introducing the battle royale shooter genre to the masses. The popularity of PUBG led Epic Games to introduce its own battle royale mode in Fortnite, a shooter that now dominates Twitch streams and has become one of the most popular games on the planet. PUBG developer Bluehole previously revealed it had “growing concerns” about the similarities between the two games, and now Korean company PUBG, a subsidiary of Bluehole, has filed a lawsuit against Epic Games in Korea.

The Korea Times reports that PUBG is alleging copyright infringement against Epic Games, and has filed a lawsuit for the Seoul Central District Court to determine whether Fortnite has copied PUBG. “We filed the suit to protect our copyright in January,” says an unnamed PUBG official in a statement to The Korea Times. It’s not clear exactly how Fortnite has allegedly infringed upon the PUBG copyright, but the two games do share the same Unreal Engine 4 that was created and licensed by Epic Games.

More than 40 million copies of PUBG have been sold, and the game has also made its way exclusively to the Xbox One console. Fortnite continues to grow in popularity, thanks in part that it’s free-to-play, and the game made nearly $300 million in the month of April alone. Both games have also arrived on mobile platforms recently, aiming to make a successful transition from PC or console to phones.