BitTorrent is an ad-supported peer-to-peer program developed by Bram Cohen and BitTorrent, Inc. used for uploading and downloading files via the BitTorrent protocol. BitTorrent was the first client written for the protocol. It is often nicknamed Mainline by developers denoting its official origins. Since version 6.0 the BitTorrent client has been a rebranded version of µTorrent. As a result, it is no longer open source. It is currently available for Microsoft Windows and Mac OS X.