Structure of micro-exon genes in the MEG-3 family. (A) Schematic representation of the position of four MEG-3 genes on a chromosome. (B) Structure of the four MEG-3 family members. Wide portions represent exons while narrow portions represent introns. Figure is to scale, but exons are depicted three times larger than introns to permit better visualization. Numbers above micro-exons indicate length in base pairs. Letters indicate homologous exons between the different members. Homology between MEG-3.1, MEG-3.2, and MEG-3.3 has been determined by genomic sequence alignment. Proposed homology between MEG-3.4 and the other genes has been determined by protein alignment, the similarity of exon positions in the gene and by their length.