The genes for the vicilin seed storage proteins of higher plants code for proteins that fall roughly into two size classes. All the vicilin genes descended from a single ancestral gene, and there is considerable homology among the amino acid sequences of all vicilin proteins except in an NH2-terminal domain that is present only in the vicilins of the large size class. This domain, composed largely of charged residues, has diverged greatly. Its nucleotide composition in all species is high in G + A (sense strand) and the domain contains long polypurine.polypyrimidine tracts. Slippage in replication due to non-B DNA structures (for example, H-DNA) resulting from these tracts is considered to be the cause of the rapid divergence of this domain.