There are other, higher levels of regulation that occur here and I started to talk about them when I mentioned our RB protein. So, control of the cell cycle occurs on a higher level with a couple of key proteins in addition to RB. One of the main proteins we talk about that regulate on a very high level is p53. It's even got a nickname: it's the "guardian of the genome." It's so important, actually, that Science Magazine called it, "the molecule of the year" in 1993. Theobromine, the main molecule that's in chocolate, hasn't even gotten this honor yet. So it shows you how important p53 is. It's more important than chocolate. P53 will bind DNA directly to produce proteins that block the progression of the cell cycle. One of those proteins include p21. P21 will function to inhibit CDK. So the CDK will not be able to activate DNA replication or activate mitosis. RB is another protein that is associated with the function of P53, and these proteins are considered tumor suppressor genes, so RB is a protein that is produced from a tumor suppressor gene, just like p53, so I'll write that down here. These proteins are considered tumor suppressor genes... They're made from tumor suppressor genes I should rather say. These proteins are made from tumor suppressor genes, which are important to have because if they are defected, or if they have a mutation in them that makes them have loss of function, so that's an important term, (repeats) defected or if they have a loss of function, what ends up happening is that you tend to get cancer. Cancer, which I think you and I can both agree is not a good thing to have. So, it is very important to have tumor suppressor genes. To illustrate how important that is, if you look at p53, greater than 50% of tumors have a defect in p53. RB got its name because a defect in RB would lead to a tumor of the eye, known as retino-that's where you got the R- retinoblastoma, which is why these two proteins are considered tumor suppressor genes. P21 is very unusual in that it doesn't actually lead to cancer when it's defected. Instead, scientists have found that mice that are without p21 have the ability to regenerate their limbs. How weird is that?! P21 lack causes the ability to regenerate their limbs, their arms and legs. So we're all still trying to figure out exactly how some of these key proteins work. But it's important to understand that these tumor suppressor genes are essential for making sure that we regulate the cell cycle so we don't just go to the divide, divide, divide phase. We want to make sure that we only divide when it's appropriate, when a cell is ready to, or if it's even a cell that is supposed to divide in the first place. Because, as we'll see in our next couple of videos, mitosis, or cell division, requires a lot of energy and protein machinery to happen. So we need to make sure it's the right time to divide.