The following are fouls, as set out by the Nevada State Athletic Commission:
-Putting a finger into any orifice or into any cut or laceration on an opponent.
-Clawing, pinching or twisting the flesh.
-Grabbing the clavicle.
-Kicking the head of a grounded opponent.
-Kneeing the head of a grounded opponent.
-Stomping a grounded opponent.
-Spiking an opponent to the canvas on his head or neck.
-Holding the shorts or gloves of an opponent.
-Spitting at an opponent.
-Engaging in an unsportsmanlike conduct that causes an injury to an opponent.
-Using abusive language in the ring or fenced area.
-Attacking an opponent on or during the break.
-Attacking an opponent who is under the care of the referee.
-Attacking an opponent after the bell has sounded the end of the period of unarmed combat.
-Flagrantly disregarding the instructions of the referee.
-Timidity, including, without limitation, avoiding contact with an opponent, intentionally or consistently dropping the mouthpiece or faking an injury.
-Interference by the corner.
-Throwing in the towel during competition. (forfeit)
Throwing in the towel is illegal (a foul), but the penalty is dq, which is what the point is. I think it's always a dq to throw in the towel which is why people do it.