Genomic structures for the two subfamilies of maize SUN-domain protein genes. The locations of exons, start (ATG), and stop (TGA, TAA) codons are shown for each gene. The diagrams were drawn from predictions made by the SPIDEY program http://www.ncbi.nlm.nih.gov/spidey/ on the basis of alignments of cDNA to genomic DNA sequences (from Table 1). The mRNA coordinates for the exon bases are listed above the diagrams. Exons are numbered, and the intron lengths (bp) appear below the diagrams. (A) The canonical C-terminal SUN domain genes show a large intron at a conserved location interrupting the SUN domain region (yellow box) within the ORF. (B) The plant-prevalent mid-SUN 3 transmembrane genes all share a large exon that contains the entire SUN domain plus a domain of unknown function (black box) associated with these genes, as well as two small introns before the last exon.