Activate the following cheats by entering the corresponding codes at "Mission Selection Screen" > "Extras" > "Secret Code":

CHEAT FUNCTION - CHEAT CODE
Master Control (go alien to alien with no timer) - Cannonbolt, Heatblast, Wildvine, 4arms
Invincibility - XLR8, Heatblast, Wildvine, 4Arms
All combos - Cannonbolt, Heatblast, 4arms, Heatblast
All locations on map - Heatblast, XLR8, XLR8, Cannonbolt
All Alien forms - Wildvine, 4arms, Heatblast, Wildvine
Dark Heroes (appearance will be darker) - Cannonbolt, Cannonbolt, 4arms, Heatblast
DNA Force (appearance will be translucent) - Wildvine, Fourarms, Heatblast, Cannonbolt