SMG5 is an essential component of the nonsense-mediated mRNA decay (NMD) pathway, functioning as a key regulator of mRNA quality control 1. The protein forms a heterodimer with SMG7 that recognizes phosphorylated UPF1, specifically binding to phosphoserine residues in UPF1's C-terminal tail through its 14-3-3-like domain 2. This SMG5-SMG7 complex serves as an adapter linking UPF1 to protein phosphatase 2A (PP2A) to promote UPF1 dephosphorylation and connects the NMD machinery to exonucleolytic mRNA degradation pathways 1. Importantly, SMG5 is required for SMG6 endonucleolytic activity, establishing functional interdependence between NMD degradation branches 3. SMG5 can functionally substitute for SMG7, and either factor is sufficient to enable SMG6-mediated endonucleolysis of NMD targets 3. The protein is subject to microRNA regulation, with miR-433 targeting SMG5 mRNA to modulate NMD activity 4. SMG5 shows elevated expression in multiple cancers, particularly hepatocellular carcinoma, where it influences therapeutic resistance and cell proliferation 56. Beyond NMD, SMG5 is necessary for telomerase activity and involved in telomere maintenance, highlighting its diverse cellular functions.