Plays a role in nonsense-mediated mRNA decay (NMD) (PubMed:16467471, PubMed:17984081). Recruits UPF1 to cytoplasmic mRNA decay bodies (P-bodies) (PubMed:17984081). Negative regulator of gene expression. Inhibits translation most likely through effects on eIF-4E (CDC33) (PubMed:16467471). Involved in telomere maintenance (PubMed:10688642). Interacts with NMD helicase UPF1 (PubMed:17984081). Interacts with CDC33 (PubMed:16467471). Results in stabilization of NMD targets. Belongs to the EST1 family.