The alien Mothership is a boss in Duke Nukem Forever.
The Mothership is a huge city sized flying fortress that has three huge laser guns attached to each side. It is probably responsible for carrying the majority of the aliens to Earth. It can house a limitless amount of Alien Fighters and Dropships.
The only means of attack comes in the form of the three powerful laser guns; however only one is in Duke's direction and is no threat till enough damage has been dealt to the Mothership. Initially none of the guns attack Duke as the ship itself is busy attacking various structures in the area , however if the Mothership is not destroyed quick enough when it's health has been lowered significantly, the laser facing Duke's direction will fire at him causing massive damage; as soon as that laser begins to target Duke it needs to be destroyed immediately as Duke cannot avoid it due to being in a turret and unable to get out of it during the battle and it only takes a couple of seconds for the laser to kill Duke if it starts firing at him. The laser gun facing Duke is the only weak spot for the Mothership as it's the only part that Duke's turret can target.
It should be noted that until the Mothership's health is lowered enough it's no threat to the player; the real concern up until that point is the Alien Fighters and Dropships that are deployed while you attack the mothership. The Dropships will drop off Assault Troopers while the Alien Fighters swing by and shoot Duke with lasers. As the health of the Mothership gets lower, Alien Fighters will appear a lot more frequently and instead of being deployed one at a time, they'll come in groups of three; however only the one in the middle of the group of three poses a threat; the other two don't attack Duke and are more of a diversion to distract the player. The Alien fighters are perhaps the largest threat for most of the battle and while they only fire a single shot before flying over Duke it can put him to the brink of death because if Duke's ego doesn't recover one more hit from another Alien Fighter will kill him among other things. The Alien Fighters can easily hit the player if they aren't paying attention to their approach or if the turret gun has to cool down before firing again.