States during 2008. More than 60 million downloadable tracks have been purchased across the series as of February 2010. Both Guitar Hero III and World Tour were listed on a March 2011 list from the NPD Group of top-grossing games in unadjusted sales in the United States since 1995; Guitar Hero III tops the list with total sales of $830.9 million. Overall, the Guitar Hero series has sold more than 25 million units worldwide, earning US$2 billion at retail. Activision claimed the series to be the 3rd largest game franchise in 2009 after the Mario and Madden NFL franchises. Cultural impact The Guitar Hero series has made a significant cultural impact, becoming a "cultural phenomenon". The series has helped to rekindle music education in children, influenced changes in both the video game and music industry, has found use in health and treatment of recovering patients, and has become part of the popular culture vernacular. Several journalists, including 1UP.com, Wired, G4TV, the San Jose Mercury News, Inc., The Guardian, and Advertising Age, considered Guitar Hero to be one of the most influential products of the first decade of the 21st century, attributing it as the spark leading to the growth of the rhythm game market, for boosting music sales for both new and old artists, for introducing more social gaming concepts to the video game market, and, in conjunction with the Wii, for improving interactivity with gaming consoles. Guitar Hero has seen a revitalization in the form of a software clone called Clone Hero, as observed in 2020. Legal and practical issues PlayStation 3 incompatibility Sony's PlayStation 3 console has no compatibility with the PlayStation 2 Guitar Hero controller on the system. While Guitar Hero and Guitar Hero II are fully backward-compatible through the hardware PlayStation 2 emulation in the initial North American release of the console, it was impossible at launch to use the guitar controller to play either game. Kai Huang, of RedOctane, states that they are "working on that with Sony right now – looking at how we can get all the PlayStation 2 guitars that are out there, and all the owners of them, to use them on the PlayStation 3." Nyko, an accessories company, was poised to make a special PlayStation 2 controller adapter for the PlayStation 3, but put the product on hold due to technical difficulties. Tac, another accessories company, also made a PlayStation