Required for pre-mRNA splicing as component of the spliceosome. Core component of the splicing-dependent multiprotein exon junction complex (EJC) deposited at splice junctions on mRNAs. The EJC is a dynamic structure consisting of core proteins and several peripheral nuclear and cytoplasmic associated factors that join the complex only transiently either during EJC assembly or during subsequent mRNA metabolism. The EJC marks the position of the exon-exon junction in the mature mRNA for the gene expression machinery and the core components remain bound to spliced mRNAs throughout all stages of mRNA metabolism thereby influencing downstream processes including nuclear mRNA export, subcellular mRNA localization, translation efficiency and nonsense-mediated mRNA decay (NMD). Binds spliced mRNA in sequence-independent manner, 20-24 nucleotides upstream of mRNA exon-exon junctions. Identified in the spliceosome C complex. Component of the mRNA splicing-dependent exon junction complex (EJC), which contains at least casc3, eif4a3, magoh, nxf1 and rbm8a (By similarity). Forms homooligomers (By similarity). Shuttles between the nucleus and the cytoplasm in a xpo1/crm1-dependent manner. Transported to the cytoplasm as part of the exon junction complex (EJC) bound to mRNA (By similarity). In the dendrites of hippocampal neurons, localizes to dendritic ribonucleoprotein granules (By similarity). Belongs to the CASC3 family.