Graphical user interfaces first appeared due in part to research taking place in the early 1970s at Xerox PARC research facility. The first CUI appeared on the Xerox Alto computer in 1973. However, graphical interfaces became more widespread with the advent of Apple Macintosh computers in the 1980s. The user interface for the Macintosh operating system (Mac OS) has undergone various changes over the years, the most significant being the adoption of the Aqua interface that appeared with Mac OS X. Microsoft's first version of Windows-Version 1.0-was based on the addition of a CUI interface to the MS-DOS operating system. Later versions of Windows have made cosmetic changes in the appearance of the CUI along with several enhancements in its functionality, including Windows Explorer.