Required for pre-mRNA splicing as component of the spliceosome. Plays a redundant role with MAGOH in the exon junction complex and in the nonsense-mediated decay (NMD) pathway. Component of the pre-catalytic, catalytic and post-catalytic spliceosome complexes. 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. Belongs to the mago nashi family.