start codon with a leucine amino acid. Leaky scanning: used by various viruses in all Baltimore groups, the 40S ribosomal subunit may scan through a start codon, thereby skipping an ORF, only initiating translation with the 60S subunit at a subsequent start codon. Ribosomal shunting: used by various dsDNA, +ssRNA, -ssRNA, ssRNA-RT, a dsDNA-RT viruses, ribosomes will start scanning from a 5′-cap structure then bypass a leader structure in the mRNA, initiation translation downstream from the leader sequence. Termination-reinitiation: used by some dsRNA and +ssRNA viruses, ribosomes may translate an ORF, but following termination of translation of that ORF, a proportion of 40S subunits of the ribosome remain attached to the mRNA as a way to reinitiate translation of a subsequent ORF. Non-canonical elongation and termination of translation: Ribosomal frameshifting: used by various dsDNA, dsRNA, +ssRNA, and ssRNA-RT viruses, produces merged proteins from overlapping ORFs. This is executed simply by ribosomes slipping one nucleobase forward or backward during translation. Suppression of termination: also called stop-codon readthrough, used by various dsRNA, +ssRNA, and ssRNA-RT viruses, certain viruses contain codons in their mRNA that would normally signal for termination of translation upon being recognized by a release factor but are instead partially recognized by tRNA during translation, which allows for continued translation up to the next stop codon in order to produce an extended end of the viral protein. In viruses, this is often used to express replicase enzymes. Ribosomal skipping: also called stop-carry on, used by various dsRNA and +ssRNA viruses, a viral peptide, or amino acid sequence, may prevent a ribosome from covalently linking a new inserted amino acid, which blocks further translation. Consequently, the polyprotein is co-translationally cleaved, and a new amino acid sequence is started, leading to the production of two individual proteins from one ORF. History Baltimore classification was proposed in 1971 by virologist David Baltimore in a paper titled Expression of Animal Virus Genomes. It initially contained the first six groups but was later expanded to include group VII. Because of the utility of Baltimore classification, it has come to be used alongside standard virus taxonomy, which is based on evolutionary relationships and governed by the International Committee on Taxonomy of Viruses (ICTV). From the 1990s to the 2010s, virus taxonomy used a 5-rank system ranging from order to species with Baltimore classification used in conjunction. Outside of the ICTV's official framework, various supergroups of viruses