And then we form carbon skeleton intermediates, and then those are used for energy purposes. But of course, every time we metabolize amino acids, we form nitrogen as a byproduct. More specifically, we form ammonium. And as this process continually takes place, we build up the amount of ammonia that is present inside our skeleton muscle cells. Now, ammonium is toxic, and so what the skeleton muscle cells must do is they must be able to dispose of that ammonium. Now, unlike in the liver, and to a smaller extent in the kidney, where we have the urea cycle to basically dispose of that ammonium inside the skeleton muscle cells, we don't actually have a way to dispose of ammonium directly.