Summary Comparison of the predicted secondary structures of the coat proteins of the potexviruses potato X and papaya mosaic, on the basis of a combination of prediction methods, shows that although the proteins differ at their amino termini, and possibly at their carboxyl termini, they are similar in the distribution of α-helical segments in the central regions of the protein chains. There is an approximate twofold symmetry in the distribution of predicted α-helical segments between the amino and carboxyl halves of the molecules.