The domain structure of expansins and a comparison with that of distantly related single-domain plant proteins (G2A, p12 and barwin). The expansin signal peptide (SP) is 20-30 amino acids long, domain 1 is 120-135 amino acids, and domain 2 is 90-120 amino acids. Some barwin proteins have an additional chitin-binding domain after the signal peptide (not shown). The positions of the introns that are present in more than one expansin family are indicated by lettered triangles; homologous introns are present in p12 and barwin proteins. Intron letters are as in . The position of intron B suggests that it could have participated in exon shuffling.