Alien Syndrome (Game Gear)

Alien Syndrome
Developer: SIMS Publishers: Sega (US/EU), SIMS (JP) Platform: Game Gear Released in JP: March 19, 1992 Released in US: 1992 Released in EU: 1992 This game has a hidden sound test. This game has a hidden level select.

Hidden Menus

At the title screen, press 1, 2, 1, 2 for this hidden menu, which has only settings for skill level and a sound test.

(Source: SMSPower)

Or, press 2, 2, 2, 2, 2, 1, 1, 2, 2, 2, 2, 2, 1, and then 1+2 for such additions as stage select, lives, captives per stage, and starting weapon.

(Source: GameFAQs (Mezmorize99))