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. Ubiquitous. Detected in brain, heart, liver, lung, spleen and testis. In macrophages, strongly induced by bacterial lipopolysaccharide (LPS). Levels reach a maximum 6 hours after exposure to LPS and are lower, but still much increased after 12 and 24 hours. Belongs to the mago nashi family. Extended N-terminus.