The TFIID–TFIIA–TBP initiation complex is formed by many subunits that come together in a non-symmetric way to carry out a specific function. Ferritin, alcohol dehydrogenase and the Satellite Tobacco Necrosis Virus coat protein are examples of symmetric homooligomers, i.e. they comprise repeating subunits that form the functional unit. Each protein chain (subunit) in each example has a unique colour to highlight each component. PDB ID from left to right: 6MZM, 1HRS, 2OHX, 2BUK.