text: The usage of multiple reading frames leads to the possibility of overlapping genes; there may be many of these in viral, prokaryote, and mitochondrial genomes. Some viruses, e.g. hepatitis B virus and BYDV, use several overlapping genes in different reading frames.
  In rare cases, a ribosome may shift from one frame to another during translation of an mRNA (translational frameshift). This causes the first part of the mRNA to be translated in one reading frame, and the latter part to be translated in a different reading frame. This is distinct from a frameshift mutation, as the nucleotide sequence (DNA or RNA) is not altered—only the frame in which it is read.