How does the defense label on Mutant Protectors work since the card ability makes an ally the defender?

When a player plays Mutant Protectors, that player becomes the target of the enemy attack and the X-Men ally put into play becomes the defender. If the defending ally leaves play before damage is dealt for the attack, the player’s hero becomes the defender and can trigger “after you defend” responses after the attack resolves. (This is not a basic defense, and the villain’s attack is not reduced by the hero’s DEF.)