FIGURE 5-18 Multiple reading frames in an mRNA sequence. If translation of the mRNA sequence shown begins at three different upstream start sites (not shown), then three overlapping reading frames are possible. In this example, the codons are shifted one base to the right in the middle frame and two bases to the right in the third frame, which ends in a stop codon. As a result, the same mRNA nucleotide sequence can specify different amino acids. Although regions of sequence that are translated in more than one of the three possible reading frames are rare, there are examples in both prokaryotes and eukaryotes, and especially in their viruses, in which the same sequence is used in two alternative mRNAs expressed from the same region of DNA, and the sequence is read in one reading frame in one mRNA and in an alternative reading frame in the other mRNA. There are even a few instances in which the same short sequence is read in all three possible reading frames.