MAGOH, the human homolog of Drosophila mago nashi, is required for embryo development. MAGOH is ubiquitously expressed in adult tissues. It has an unusual structure consiting of an extremely flat, six-stranded anti-parallel β sheet packed next to two helices. MAGOH interacts with the Y14 protein to form a complex that plays a crucial role in postsplicing processing (including nuclear export and cytoplasmic localization of the mRNA) and in the nonsense-mediated mRNA decay (NMD) surveillance process. The MAGOH-Y14 complex remains persistently associated in the same position on the mRNA after its export to the cytoplasm and requires translation of the mRNA for removal. This complex may illustrate the mechanism of the pre-mRNA splicing machinery for forming a stable exon-exon junction complex-mRNA at splice junctions.