Microsoft is replacing its Skype for Business application with Teams as the company’s primary teamwork software, in a bid to defend itself against the threat of upstart Slack in the workplace collaboration software market.

“Microsoft Teams will evolve as the core communications client,” said Ron Markezich, Microsoft’s corporate veep for Office 365.

With Skype for Business and Yammer, Microsoft claims to have made it possible for professionals to work together easily online.

From 2015 to 2016, the teamwork market grew by 13.6 percent, and it is expected to be worth more than $22 billion by 2021, according to researcher IDC.

Microsoft says it will continue supporting Skype for Business, but expects subscription-based Office 365 users to migrate to Teams over time.