LPTMC, Université Paris-VI-CNRS UMR 7600, Tour 24, 4-ème, Boite 121, 4 Place Jussieu, Paris 75005, France
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A classification of icosahedral viral capsids is proposed. We show how the self-organization of capsids during their formation implies a definite composition of their elementary building blocks. The exact number of hexamers with three different admissible symmetries is related to capsids' sizes, labelled by their T-numbers. Simple rules determining these numbers for each value of T are deduced and certain consequences concerning the probabilities of mutations and evolution of viruses are discussed.