Sequence data are available for the coat proteins of seven tobamoviruses, with homologies ranging from at least 26% to 82%, and atomic co-ordinates are known for tobacco mosaic virus (TMV) vulgare. A significant spatial relationship has been found between groups of residues with identical amino acid substitution patterns. This strongly suggests that their location is linked to a particular function, at least in viruses identical with the wild-type for these residues. The most conserved feature of TMV is the RNA binding region. Core residues are conserved in all viruses or show mutations complementary in volume. The specificity of inter-subunit contacts is achieved in different ways in the three more distantly related viruses.