Primary

• Sequence of amino acids in a protein chain.
• There are 20 amino acids, so at every point in a chain, there are 20 alternatives.
• Most proteins are at least 100 amino acids long so there are over $20^{100}$ possible ways of ordering amino acids.
• The order of amino acids in the primary structure will determine the shape of the protein molecule through its secondary, tertiary and quaternary structure.