Component of the telomerase ribonucleoprotein (RNP) complex that is essential for the replication of chromosome termini (PubMed:19179534). May have a general role in telomere regulation (PubMed:12676087, PubMed:12699629). Promotes in vitro the ability of TERT to elongate telomeres (PubMed:12676087, PubMed:12699629). Overexpression induces telomere uncapping, chromosomal end-to-end fusions (telomeric DNA persists at the fusion points) and did not perturb TRF2 telomeric localization (PubMed:12676087, PubMed:12699629). Binds to the single-stranded 5'-(GTGTGG)(4)GTGT-3' telomeric DNA, but not to a telomerase RNA template component (TER) (PubMed:12676087, PubMed:12699629). {ECO:0000269|PubMed:12676087, ECO:0000269|PubMed:12699629, ECO:0000269|PubMed:19179534}. Plays a role in nonsense-mediated mRNA decay (PubMed:17053788, PubMed:18974281, PubMed:19060897, PubMed:20930030). Is thought to provide a link to the mRNA degradation machinery as it has endonuclease activity required to initiate NMD, and to serve as an adapter for UPF1 to protein phosphatase 2A (PP2A), thereby triggering UPF1 dephosphorylation (PubMed:17053788, PubMed:18974281, PubMed:19060897, PubMed:20930030). Degrades single-stranded RNA (ssRNA), but not ssDNA or dsRNA (PubMed:17053788, PubMed:18974281, PubMed:19060897, PubMed:20930030). {ECO:0000269|PubMed:17053788, ECO:0000269|PubMed:18974281, ECO:0000269|PubMed:19060897, ECO:0000269|PubMed:20930030}. This is the function of SMG6 (SMG6 nonsense mediated mRNA decay factor, ENSG00000070366).