(A) In a normal mRNA molecule (blue and purple rectangle), the ribosome (brown ovals) scans along the coding region (purple), and translates a protein (brown line); this process stops when the ribosome encounters a stop codon (stop sign). The mRNA after the stop codon is called the 3′ untranslated region (3′UTR, blue), and does not encode a protein. (B) In EJC-nonsense-mediated decay, if a nonsense mutation is located more than 50 nucleotides upstream of an exon junction complex (EJC; green circles), this is recognized as aberrant and the mRNA is degraded. (C) Zhao et al. have discovered a new method of nonsense-mediated decay that depends on the regulatory actions of microRNA molecules. The coding region downstream of a nonsense mutation behaves as 3′UTR. If a microRNA binding site is present in this region, the microRNA binds to it, which inhibits translation and leads to the mRNA molecule being degraded.