The world’s leading repository of open-source code is set to become part of a software giant that built its fortune on proprietary software. Some developers are already worried.

The news: Bloomberg first reported news of the transaction, and Microsoft has now confirmed it’s buying GitHub, the leading online service that lets developers store and collaborate on code development. GitHub began by focusing on open-source software but has since branched into services that let large companies like Facebook use it for more sensitive, proprietary projects.

The prize: GitHub is used by over 28 million developers and holds over 85 million “repositories” of code. It’s popular because of its Git software-management system, which makes tracking different versions of software dead easy. It also benefits from a powerful network effect—as more developers have joined it, GitHub has become even more attractive to other coders.

Why this matters: Microsoft says it’s been the most active organization on GitHub, and that it wants it to operate independently. But why, then, is it paying so much for the business, which was last valued at $2.5 billion in 2015?

The most plausible answer is that the company will significantly influence the way the platform operates to its advantage, and get an invaluable overview of the projects being worked on across it. That will propbably spook big software companies like Google and Facebook, which won’t like the idea of developing their code on a platform controlled by a rival. Some coders are already expressing misgivings about the deal as well. Unless Microsoft treads carefully, some could flee to rival repositories like BitBucket or GitLab, which has seen a big spike in traffic since news of the deal broke.