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? And each one of these amino acids can basically be any one of those 20 amino acids. So, to carry this out, we have to use a bit of mathematics and probability. So let's begin with amino acid number one.