Eukaryotic nonsense-mediated mRNA decay. (a) The spliceosome deposits exon junction complexes (EJCs) at the splice sites of an mRNA (see Chapter 16), and the EJCs are usually displaced by the ribosome as it travels along the mRNA. (b) If the ribosome encounters a premature stop codon upstream of an EJC, then Upf1 and Upf2 recruit a decapping enzyme and a 5′→3′ exonuclease to promote degradation of the mRNA.