Pokémon Blue is the third core series Pokémon game released as a minor revision of Pokémon Red and Green, which were released earlier that year. It was thus the first solitary version in the core series of Pokémon games. Various fixes in the game include a graphics and sound upgrade, as well as the removal of several known glitches that had been found in the original pair. Outside of Japan, its graphics, game engine and script formed the basis of Pokémon Red and Blue, while the wild Pokémon and game-exclusive Pokémon lists were changed to match Red and Green.