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Ninja Gaiden (忍者外伝) is a 1991 side-scrolling action game published by Sega for the Game Gear. It is based on the series of the same name originally produced by Tecmo, this is an original creation by a different developer. Despite the "reprogrammed by Sega" line on the title screen, this game is not directly based on any previous Ninja Gaiden title.
While the Ninja Gaiden series was traditionally known as Ninja Ryūkenden (忍者龍剣伝) in Japan and Shadow Warriors in Europe, this game was released under the Ninja Gaiden name in all regions. Strangely, a completely different game also called "Ninja Gaiden" was released for the Master System around the same time.