Do you want to find the DNA of Mewtwo? Then listen up. Remember the guy in Cerulean that had the itemfinder that kept going off? You probably haven't found the item yet. You find it on the water, so have a Pokemon
with surf. Surf to the area two spaces away from the itemfinder guy. You should be on the same line as him.
Surf left in a straight line until there is one space between you and the left rock. Click A and you will get the Berserk Gene. You give it to your Pokemon like a berry (holding it). In a battle, it will kick in immediatly. The great thing is that it DOUBLES your Pokemon's ATTACK stat. The bad things are that it confuses your Pokemon and after one battle, the Berserk Gene will be gone. There is only ONE in the game so use wisely OR clone.