Plays a role in nonsense-mediated mRNA decay (By similarity). Recruits UPF1 to cytoplasmic mRNA decay bodies (By similarity). Together with SMG5 is thought to provide a link to the mRNA degradation machinery involving exonucleolytic pathways, and to serve as an adapter for UPF1 to protein phosphatase 2A (PP2A), thereby triggering UPF1 dephosphorylation (By similarity). Required for normal embryonic development (PubMed:19414594). Expressed during early cleavage, gastrulation and at 1 day post-fertilization. Morpholino knockdown leads to a phenotype ranging in severity from weak to severe that includes an elongated hindbrain, altered midbrain-hindbrain boundary, stacked somites in severe phenotypes and a mortality rate of 97% at 5 days post-fertilization.