Required for pre-mRNA splicing as component of the spliceosome (PubMed:28502770, PubMed:29301961, PubMed:30705154). Plays a redundant role with MAGOH in the exon junction complex and in the nonsense-mediated decay (NMD) pathway (PubMed:23917022). Component of the pre-catalytic, catalytic and post-catalytic spliceosome complexes (PubMed:28502770, PubMed:29301961, PubMed:30705154). Heterodimer with RBM8A. Core component of the mRNA splicing-dependent exon junction complex (EJC); the core complex contains CASC3, EIF4A3, MAGOH or MAGOHB, and RBM8A (PubMed:23917022). Ubiquitous. Belongs to the mago nashi family.