Plays a role in nonsense-mediated mRNA decay (By similarity). Does not have RNase activity by itself (By similarity). Promotes dephosphorylation of UPF1 (By similarity). Together with SMG7 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). Necessary for TERT activity (By similarity). Required for normal embryonic development (PubMed:19414594). Expressed during early cleavage, gastrulation and at 1 day post-fertilization. Morpholino knockdown leads to phenotypes ranging in severity from weak to severe that are developmentally delayed with disturbed brain patterning, necrosis in areas of the brain, aberrant eye development, impaired somitogenesis resulting in stacked somites, perturbed yolk sac extension and posterior axis extension with a high mortality rate of 88% at 5 days post-fertilization.