So whenever we're conducting some type of assay there are two questions we have to ask ourselves. Question number one is, is the protein present in our sample? And the way that we answer the question is by measuring the protein activity or if we're dealing with an enzyme, which is usually the case, we measure the enzyme activity of that protein and we'll see exactly how that's done in just a moment. And the second question is, if that protein is present and we can measure the enzyme activity, what is the concentration of that protein? What is the amount of protein found inside that sample? So once we know the enzyme activity and the concentration of that protein that are sampled, we can then calculate the specific activity.