And there are five common ways by which our cells modify amino acids, as we'll see in just a moment. Actually, there are many more ways, but we're going to focus on these five in this lecture. So we can modify amino acids by adding acetal groups, hydroxyl groups, carboxyl groups, sugar groups, phosphoryl groups, as well as many other different types of groups which we're going to discuss in future lectures. So in this lecture, let's focus on these five. So let's begin with the first one. So the majority of the polypeptides and proteins inside our body are actually modified by the addition of acetyl groups.