So it begins here and ends here. Now we don't know what the sequence of nucleotides in this target sequences, the target sequence of the sequence we want to amplify, but we do know where that DNA segment is found. We do know what the flanking sequences, the flanking sequence are. Basically these sequences found on both sides of that gene of interest. So the flanking sequence is a segment of DNA that we do not want to replicate, but that we know the sequence of nucleotides because we have to know what the sequence within these sections here, because we're going to build the DNA primers that can essentially hybridize with those flanking regions. Because the DNA polymerase that is used in the polymerase chain reaction needs the primus to actually synthesize the DNA molecule.