Zynga ( ZNGA ) shares were up 6.5% in midday trading Tuesday after the games maker late Monday said founder Mark Pincus has given up his role as CEO in favor of longtime gaming executive Don Mattrick. Mattrick's leaving Microsoft ( MSFT ) after six years. For the last three, he led Microsoft's Xbox 360 gaming business. "Zynga is a great business that has yet to realize its full potential," Mattrick, 49, said in a statement. The transition garnered some praise from Wall Street because Mattrick has had "some successes" with Xbox, wrote Cowen & Co. analyst Doug Creutz. Pincus will continue serving as chairman and chief product officer, and will still hold a controlling share in the company. That could later result in a power struggle, Creutz wrote in a research note.