 Nucleic acids are cool because we have these letters, the alphabet, right, A-G-C-T or A-G-C-U. And because of the complementarity, if I have a particular sequence of letters, in principle, all I need is a strand with a complementary sequence that will bind to the first one and then get it to do something that makes it possible for me to detect. One such idea is what if I have a... Leave some room under here. What if I have some sort of test sequence here? So this is some sort of test R and test sequence. Actually, the complementarity test sequence. And then I combine that with some sort of hairpin or so. So this is just going to be a number of other bases, RNA bases, that will pair up. And then down here, I add two parts. First, I add a fluorophore. So what's a fluorophore? Well, a fluorophore is going to be a molecule that exhibits fluorescence, right? So if I shine light on this one, I will get light back. But the smart thing that's right next to that one on the other side, I add a so-called quencher. So this is not a class on spectroscopy, but the idea is that if the quencher is physically close to the fluorophore, it's going to eat the energy basically, so you won't see any fluorescence. But if these two separate more than, say, 20 angstrom or so, then we're going to see fluorescence. So the idea here, what happens, is that depending on the sequence we have up here, if this one occasionally unfolds, if the particular test sequence here binds to something like the viral RNA, then this one will, instead of being in this shape and up in this shape. And now we have the fluorophore there and the quencher there and, well, maybe virus RNA or something here. And here we're now going to see fluorescence. We don't always use fluorescence for this. There are smarter chips and other techniques to do. A very typical way is so-called microarrays because these are cheap and simple to manufacture. In the case of the PCR test for COVID, of course, there's only one or a handful of specific sequences we want to test. We might want to combine the tests so we can test a few mutants at the same time. But we can have so-called microarrays like this one. And so it's probably a centimeter squared. On such a chip, you can fit in the ballpark of 500,000 different tests. And these tests are usually designed so that they test for one specific mutation. So for instance, is that particular, is that, we know what this sequence is, but I want to test in this particular genome, has that residue mutated to something? That makes it possible to use these gene chips or microarrays to test a ton of different simple diseases just by checking if there has been a mutation on a specific single site in my genome. This type of, not so much tests, but this type of mutations is usually called SNPs. So single nucleotide polymorphism. So literally mutations that is only changing one nucleotide, one base in DNA, those are extremely common and I think roughly one in general out of 1000 bases in my genome, maybe one or so has mutated pure, by pure chance. What can we use this for? Well, in addition to detecting simple stuff like virus RNA, which you can do a sewage water, two or other things, we can also do this on a large scale for testing disease. Let me show you one.