Go! Sudoku is a sudoku puzzle game for the PlayStation Portable, released in Europe on December 2, 2005 and North America on March 21, 2006. It was later released in Japan on April 27, 2006 under the name of Kazuo (カズオ), as Nikoli holds the trademark for Sudoku. The game was mildly received by gamers and professional critics. The base game features 1000 Sudoku puzzles grouped by difficulty, as well as various modes, customizable grids and multiplayer. 200 more puzzles are available for download from the Go! Sudoku official website, and players can add their own custom backgrounds to the games.