The next question is how do we identify which one of these bands contains that gene of interest? So remember in the beginning I said that we have to know what that sequence of nucleotides is in that gene that we want to isolate. And so what we do in step three is we build a DNA molecule, a DNA probe that contains a complementary nucleotide sequence that is complementary to that DNA fragment, the gene that we want to isolate found in fragment C. And when we build it, we radioactively label that DNA probe. For example, we use radioactively heavy phosphorus atoms. And what that will allow us to do is in step four, we're going to be able to use x ray autorediography to basically find exactly where that DNA molecule is. So in step three, a specific restriction fragment of interest.