Now, once again, it's important to know that within these regions, these are no longer in their double stranded form, they exist as single stranded DNA molecules. And that leads us directly into step three. So in step two, we essentially separated these fragments based on size. 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.