In this video, we're going to begin our lesson on a technique that's called southern blotting, but before we talk about the actual southern blotting technique, it's actually first helpful to talk about radioactive probes. And so in this video, we're going to focus on radioactive probes, and in our next lesson video, we'll talk more about the southern blotting technique. And so what's important to know is that after cloning a gene, the DNA that was cloned can actually be used as a probe to detect the same sequence in an unknown DNA sample. A probe or probes are really just radioactively labeled molecules that are visualized using radioactive detection. And so DNA probes are going to be single-stranded DNA molecules that are going to be complementary to a specific sequence of interest. And so the DNA probe, which is going to be single-stranded and radioactive, when it is bound to the specific sequence of interest, we can detect the presence of a specific sequence of interest by detecting the presence of the radioactive probe. And so if we take a look at our image down below, we can get a better understanding of these radioactive probes. And so here in our example, it's saying radioactive probes are used to identify DNA that is complementary to it in different samples. And so notice in this image over here on the left side, we have 2 different test tubes with different samples. We have sample A over in the left test tube and sample B over here in the right test tube, and both of these samples contain DNA. And so suppose that we want to identify which of these samples has a specific DNA sequence of interest. What we can do is we can use a radioactive DNA probe, which is going to be a single-stranded DNA molecule as you see here, which is going to be radioactively labeled. And so we can take this radioactive DNA probe and we can put it into both of these samples. And under the right conditions, when we incubate the samples with the probe, what will happen is the sample where we can actually detect radioactivity is going to have the sequence of interest, whereas the sample where we do not detect radioactivity will not have the sequence of interest. And here in this image, the radioactivity is represented by the yellow color that you see. And so in terms of having the sequence of interest, because we see radioactivity in sample A, we would say that sample A does have the sequence of interest. So we would say yes here. But sample B, because we do not detect any radioactivity, would mean that it does not have the sequence of interest. And so basically, by using these radioactive probes, we can detect the presence of a specific sequence. And so, we'll be able to see how radioactive probes are going to be important in the technique Southern blotting in our next video. But for now, this here concludes our introduction to radioactive probes and we'll be able to get practice applying these concepts as we move forward. So I'll see you all in our next video.