The multiprotein exon junction complex (EJC) is deposited on mRNAs upstream of exon–exon junctions as a consequence of pre-mRNA splicing. In mammalian cells, this complex serves as a key modulator of spliced mRNA metabolism. MLN51 is a nucleocytoplasmic shuttling protein that is overexpressed in breast cancer. The function of MLN51 in mammals remains elusive. Its fly homolog, named barentsz, as well as the proteins mago nashi and tsunagi have been shown to be required for proper oskar mRNA localization to the posterior pole of the oocyte. Magoh and Y14, the human homologs of mago nashi and tsunagi, are core components of the exon junction complex (EJC). The EJC is assembled on spliced mRNAs and plays important roles in post-splicing events including mRNA export, nonsense-mediated mRNA decay, and translation. Human MLN51 is an RNA-binding protein present in ribonucleo-protein complexes.