We have a sample of proteins. Now, where did the sample actually come from? So suppose we have a collection of cells, and inside the cells we have proteins. And the protein that we want to actually locate is found within the cells. So the way that we're going to isolate the proteins from inside the cells is by fractionating our cells and then placing that fraction into a differential centrifuge machine. And then as a result of centrifugation, we're going to basically isolate the proteins found inside our cell.