Because to build the DNA primer, we have to know what the complementary sequence is to this group here. So if this is ACG, then we have to build a primer that contains a sequence TGC. And so we can build that in a laboratory. And we also radioactively label that DNA primer because that will basically allow us to pinpoint exactly where that molecule is when we undergo the process of gel electrophoresis. So we add a label DNA primer that is complementary to the three end of that single stranded DNA molecule that we want to sequence. So this is the three end of that DNA molecule.