The mutant's mind shrinks (Int -20) and it becomes subject to stupidity. It must make a Cl test when doing anything that requires the least thought: fighting, running, going through a doorway. Failure means that the mutant stands rooted to the spot, trying to work out what is going on.
If the mutant's Int is 13 or less, it cannot understand or use weaponry. As a result, it cannot make weapon attacks.