Inhibitor chip

Clone soldiers in the Grand Army of the Republic were engineered with a special inhibitor chip to help ensure they could be controlled. Although there were reports of some defective chips causing clones to act erratically during the Clone War, the device was vital to the execution of Order 66, which brought about the end of the Jedi Order. Only a handful of clone troopers, including the members of Clone Force 99, seemed to be unaffected.