Call of Duty 4: Modern Warfare is the fourth installment of the main series, and was developed by Infinity Ward. It is the first game in the series not to be set during World War II, but set in the modern day. It is the first to receive a Mature rating from the ESRB, except for the Nintendo DS version, which was rated Teen. The game was released for Microsoft Windows, Nintendo DS, PlayStation 3, and Xbox 360 on November 7, 2007. Download and retail versions for Mac OS X were released by Aspyr in September 2008. As of May 2009, Call of Duty 4: Modern Warfare has sold over 13 million copies.