A protein subunit is a polypeptide chain or single protein molecule that assembles (or “coassembles”) with others to form a protein complex. A subunit is often named with a Greek or Roman letter, and the numbers of this type of subunit in a protein is indicated by a subscript. For example, ATP synthase has a type of subunit called α. Three of these are present in the ATP synthase molecule, leading to the designation α. Protein quaternary structure describes the number and arrangement of multiple folded protein subunits in a multi-subunit complex.