X [Model DMG-ECJ]

Released on May 29, 1992 in Japan. This cartridge is considered to be one of the most difficult game cartridges to find.

X was designed by Dylan Cuthbert of Argonaut Games, who would later program "StarFox [Model SHVC-FO]" for the Super Famicom. After being intrigued by the Game Boy's during the 1991 Consumer Electronics Show, Argonaut president Jez San commissioned Cuthbert to produce a 3D engine for the console and potentially create a game utilizing it. Cuthbert and his development team had to reverse-engineer the Game Boy hardware as official Nintendo development kits were hard to come by, and that Argonaut was virtually unknown to them. A dummy developer kit was produced with a camera pointing at the system, produced by dismantling the console itself with a Tetris cartridge inserted and connecting it to a board made by another Argonaut employee. Cuthbert began work on X once he became familiar with the hardware; he was responsible for creating some of the graphics, most of the game design, and all of the programming. Cuthbert used an Amiga 3000 for development.

The development for the game was created in the style of Argonaut's older computer games, notably Starglider 2, featuring full 360-degree movement through space. Its technological accomplishments quickly caught the eye of a Nintendo employee, who urged that they send a ROM of the game to Nintendo's headquarters in Kyoto, Japan. It was presented to both Game Boy creator Gunpei Yokoi and engineer Ishuna, who were 'blown away' by its usage of 3D wireframe graphics, requesting that Dylan and his team fly over to Japan and meet them. After being shown to other employees, Nintendo became so impressed with the game that they purchased the rights to it from the original publisher, Mindscape, and made Argonaut an official third-party licensee. Assisting development was Nintendo Research and Development 1 (R&D1), the division responsible for producing the Game Boy itself.

At first, the title was named Eclipse, but then-Nintendo President Hiroshi Yamauchi insisted upon it being renamed X, calling up director Yoshio Sakamoto a month before development was completed. Nintendo forced Cuthbert and Argonaut to rewrite the 3D engine entirely due to a game-breaking bug that made it unplayable on certain Game Boy systems, due to the company's strict stance on quality.

A North American version titled Lunar Chase was completed by Cuthbert; however, Nintendo of America felt that the idea of a 3D first-person game on a handheld for children was too complicated, and (to his disappointment) cancelled its release. Cuthbert believed that a lack of interest from retailers was also to blame.

X is the first game to feature the infamous Totaka's Song.

A DSiWare sequel, X-Scape, was released worldwide in 2010.