Now, previously we focused on proteins that consist of a single polypeptide chain and we discussed how we can sequence the amino acids in those types of proteins. But we know many proteins inside our body. For example, hemoglobin have more than one polypeptide chain. So they have have quotinary structure. And the question is, if a protein has quotinary structure, if a protein consists of two or more polypeptide chains, how do we sequence these types of proteins? So how do we determine the amino acid sequence in proteins that consists of two or more polypeptide subunits?