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. And that's because what this does is it tells our cells not to break down and degrade those polypeptide. So many proteins are acetylated at the terminal amino groups to prevent degradation by our cells. So if we go to the beginning of our polypeptide, the nitrogen is basically modified by adding this acetal group.