And the fact that we have 20 different amino acids means we have a great variety of possible sequences for any given protein. And to demonstrate what we mean by that, let's consider a protein that contains only ten amino acids. So we have amino acid one, amino acid two, amino acid three, all the way to amino acid number ten. Now, notice these amino acids are held together by covalent bonds known as peptide bonds. And we'll talk about what those are in much more detail in a future lecture. Now, the question we want to answer is how many possible sequences are there if we have these ten amino acids?