Shortly after release, "Guitar Hero" became an unexpected hit; it was the second-highest-selling PlayStation 2 title in February 2006 according to the NPD Group. Game sales amounted to $45 million in 2005. Since then, the game has sold about 1.53 million copies through December 2007. The success of the game has spawned a one billion dollar "Guitar Hero" franchise, including four sequels on several seventh generation consoles, seven expansions, a mobile phone-based version, and a portable version for the Nintendo DS. Harmonix no longer is involved in development of the series, due to its acquisition by MTV. Harmonix has since developed "Rock Band" using designs similar to those that based "Guitar Hero"'s success.