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We're making a game that uses the Game Center leaderboards and achievements. After a game ends, we have a button that presents a GKGameCenterViewController when clicked. Is it OK to use the Game Center logo (the four colored balls) as an icon for the button? We've seen the logo used in this way for some other apps (e.g., Fruit Ninja), but we couldn't find a definitive answer as to whether this was generally allowed or whether some sort of special permission and/or licensing is required.