Upf1 was identified as an active component in nonsense-mediated decay (NMD), an mRNA surveillance mechanism in eukaryotic cells that degrades mRNAs containing premature termination codons. Upf1 was found to be an ATP-dependent RNA helicase in the cytoplasm and was later shown to be a component of cytoplasmic P-bodies. Upf1 phosphorylation mediates the repression of translation that accompanies NMD, allowing mRNA accessibility to the NMD machinery. Two other active components of NMD, Upf2 and Upf3, were also identified and described as having perinuclear and nucleocytoplasmic localization, respectively.