A group of video game testers is forming Microsoft’s first labor union in the U.S., which will also be the largest in the video game industry.

Communications Workers of America said Tuesday that about 300 quality-assurance workers at Microsoft video game subsidiary ZeniMax Studios have voted to join the union.

Microsoft already told the CWA it would accept the formation of the union at its Maryland-based video game subsidiary, fulfilling a promise it made to try to build public support for its $68.7 billion acquisition of another big game company, Activision Blizzard.

Microsoft bought ZeniMax for $7.5 billion in 2021, giving the Xbox-maker control of ZeniMax’s well-known game publishing division Bethesda Softworks and popular game franchises such as The Elder Scrolls, Doom and Fallout.

The unionization campaign was sparked, in part, by Microsoft’s ongoing bid to buy California-based game giant Activision Blizzard. Microsoft, which is based in Redmond, Washington, made a June pact with the CWA union to stay neutral if Activision Blizzard workers sought to form a union.

Microsoft’s legally binding agreement specifically applied to Activision Blizzard workers after the closing of the merger. But it also reflects Microsoft’s broader principles on handling unionization, which is still uncommon in the tech and gaming industries.

“They have definitely stood by their word all along,” said CWA spokesperson Beth Allen. “It’s pretty momentous. Microsoft is an outlier in the way tech companies have been behaving.”