sentences:  In response, Activision filed a suit seeking a declaration that it was not in violation of the Gibson patent; Activision also asserted that Gibson had given an implied license by waiting to assert the patent and that the patent was invalid.On March 17, 2008, Gibson sued six retailers (GameStop, Amazon.com, Wal-Mart, Target, Toys "R" Us and Kmart) for selling "Guitar Hero" products.Subsequently, on March 21, 2008, Gibson also filed a lawsuit against EA, MTV, and Harmonix over their game "Rock Band" also for violation of its patent, to which a Harmonix spokesperson stated that Gibson's claims are "completely without merit".