Viral genomes contain fewer genes and are more compact than eukaryotic or prokaryotic genomes. Overlapping reading frames are often present in viral genomes and this enables multiple proteins to be made from a single transcript. This genomic layout can serve different purposes: more efficient use of the genome; fewer resources needed for making transcripts; and the abundance of each protein can be optimized by regulating translation from one transcript. Programmed ribosomal frameshifting is a mechanism by which viruses can express overlapping reading frames.