In the late 1970's Microsoft licensed Unix source code from AT&T, which at the time was not licensing the name UNIX. Therefore, Microsoft created its own name called: Xenix. Microsoft never sold Xenix directly to end-users and relied on OEMs such as Intel, Tandy, Altos, and SCO to provided a customized version of Xenix to their customers. After the AT&T breakup, AT&T sold UNIX and in 1987 Microsoft transfered its ownership to SCO who continued to support the product and later released their own version called SCO UNIX.
Also see: Unix