MachTen is based on 4.4BSD and the Mach kernel, and features the X Window System and GNU programming tools. MachTen development started in 1989, culminating in the first release in 1991. The Professional MachTen branch, intended for 68000-based Macintoshes, ended with release 2.3. The Power MachTen branch, which was Power Macintosh compatible, lacked some of the features of Professional MachTen (including true virtual and protected memory models), but took full advantage of the PowerPC processor and was compatible with Mac OS 9 through its final version, 4.1.4. MachTen is no longer developed, as it was largely superseded by Mac OS X.