MATE is a fork of GNOME 2 created due to dissatisfaction amongst some users with the new approach to the desktop used by GNOME 3 and the drop of support for GNOME 2. MATE uses a more traditional approach to the desktop, versus GNOME Shell or Unity.
MATE uses GTK+ 2 at the moment, but might in the future also offer support for GTK+ 3. Otherwise, it tries to stay as close to GNOME 2 as possible.
The only Linux distribution which offers MATE as a default desktop environment at the current time is Linux Mint. MATE has not yet been formally released.
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