So we saw that the first amino group that ends up in the urea came from this free ammonia. But the second amino group that ends up on the urea comes from aspartate. Now, once we form arginosuxanate, that is then broken down into arginine and fumarate. So what happens in step four? So ultimately, the second amino group that came from aspartate ends up on the arginine when this reaction happens, but the carbon skeleton that was found on the aspartate essentially ends up on the fumerate. In fact, the carbon skeleton of the aspartate is fumerate.