And finally, in the previous lecture we discussed tertiary structure and we said that tertiary structure is the interaction, it's the spatial arrangement of those amino acids that are found far away on that polypeptide chain. In this lecture we're going to focus on the final level of protein structure. So all proteins contain primary structure. The majority of proteins contain secondary and tertiary structure and some proteins also contain a fourth and final level of structure known as quaternary structure. So a protein is said to have quarter structure if that protein actually consists of two or more individual polypeptide chains and the quarterliary structure is basically the interaction of these polypeptide chains with respect to one another. Now, the simplest type of quaternary structure is a dimer.