From January 1993, the National Center for Supercomputing Applications (NCSA) at the University of Illinois, provided pre-releases of its Mosaic browser for the Unix X Window System. The first official release was on 21 April 1993. Mosaic quickly gained popularity, becoming the browser of choice, with its user-friendly graphical interface and easy installation. Versions of Mosaic running on PC and Mac became available later that year.