An analytical view of the soybean expansin gene superfamily. The following parts are shown from left to right. Protein neighbor-joining tree: The unrooted tree was constructed using MEGA v5.0. The expansin proteins are named from their gene name (see Table 1). Gene structure: The gene structure is presented by green boxes that correspond to exons, and linking black lines that correspond to introns, while the blue line refers to the 5′-UTR and 3′-UTR. Motif compositions: The colored boxes represent the motifs in the protein, a total of 10 types of motifs were found in these 75 expansin genes, as indicated in the table on the right-hand side. The scale at the top of the image may be used to estimate motif length. aa, amino acids. A detailed motif introduction is shown in Additional file 9.