LookAt mouse, with run animation

So basically I have the player running(in a sidescroller) and I
want them to be pointing a gun at the mouse while playing the run animation.
They will also need to jump and the gun shouldn’t be pointing during that.

I would appreciate even the theory for doing this, though a full solution
would be better.

EDIT: What I mean is that when you import a model, the bones are imported as well, however they no longer affect the mesh, so simply adding lookAt to the shoulder doesnt work. I want the lookAt (or similar) to affect the mesh in some way, but I don’t know where to start.

Thanks in advance

If I understand you correctly, you need to point the gun depending upon where the mouse is?

For this try creating an animation with the full range of motion. Set the animation to a certain position within the clip, depending upon where the mouse is.

Then you can blend the other animations in with layering.