Microsoft announced the acquisition of 6Wunderkinder, the German startup behind the Wunderlist to-do list app.

While the price was not disclosed, The Wall Street Journal (WSJ) estimates the deal to be worth between $100 million and $200 million.

“The addition of Wunderlist to the Microsoft product portfolio fits squarely with our ambition to reinvent productivity for a mobile-first, cloud-first world,” wrote Eran Megiddo, general manager of OneNote for Microsoft, in a blog post.

“Building on momentum for Microsoft Office, OneNote and Skype for Business, as well as the recent Sunrise and Acompli acquisitions, it further demonstrates Microsoft’s commitment to delivering market leading mobile apps across the platforms and devices our customers use – for mail, calendaring, messaging, notes and now tasks,” he added.

Microsoft recently acquired the makers of Acompli, a mobile email app, rebranding it as Outlook for mobile, as well as Sunrise Atelier, the company behind the Sunrise mobile calendar app.

The three acquisitions reveal Microsoft’s strategy to integrate its productivity tools with a particular focus on apps, WSJ quoted an anonymous source familiar with the deals as saying.

Last month, 6Wunderkinder partnered with Sunrise so that users could see their to-do lists and add to them in the Sunrise app.

“Joining Microsoft gives us access to a massive wealth of expertise, technology and people that a small company like us could only dream of amassing on its own,” said 6Wunderkinder CEO and founder, Christian Reber.

The acquisitions are not the only way Microsoft has been busy with apps. It is bringing Cortana and its Office apps to iOS and Android and last month it unveiled a raft of updates at its Build 2015 event, including new tools to enable Android and iOS developers to take code to Windows 10.

Meanwhile, a recent Strategy Analytics report has suggested that app developers are less enthusiastic about supporting Windows Phone because of the “dominance and ecosystem extensions offered by iOS and Android”.