With the invention of the Intel 4004 processor in 1971, it became that computers could be miniaturized and shrink in size. Although the Alto did not integrate the Intel chip (it had a custom-built TTL 16-bit CPU), it was the first desktop computer that would eventually drive mainframe toward extinction. The Alto, developed by and used at the famed Xerox Palo Alto Research Center (PARC) was first to use a graphical UI, and came with a 3-button mouse. It was never sold commercially, even if a slightly modified version, the Star, was sold in 1981 for $17,000. A total of 2,000 units were built for Xerox as well as academia.