At that time, Kojima produced "Boktai"s sequel, "" for the Game Boy Advance. Released in summer 2004, it makes more extensive use of the cartridge's sunlight sensor and allows players to combine various new solar weapons. Also released was "Metal Gear Acid" for the PlayStation Portable handheld. A turn-based game, it is less action-oriented than the other "Metal Gear" games and focuses more on strategy. It was released in Japan on December 16, 2004. Its sequel, "Metal Gear Acid 2", was released on March 21, 2006.