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Class 4: Single-stranded RNA viruses - positive-sense edit Positive-sense RNA viruses, along with all genes categorized as positive-sense, have the capability to be directly engaged by host ribosomes, leading to immediate protein synthesis.Class 5: Single-stranded RNA viruses - negative-sense edit Negative-sense RNA viruses, along with all genes classified as negative-sense, cannot be directly utilized by host ribosomes to synthesize proteins instantly.

Class 4: Single-stranded RNA viruses - positive-sense
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Positive-sense RNA viruses, along with all genes categorized as positive-sense, have the capability to be directly engaged by host ribosomes, leading to immediate protein synthesis. These viruses can be categorized into two distinct groups, both of which undergo replication within the cytoplasm:
Viruses possessing polycistronic mRNA, where the RNA of the genome acts as the mRNA, allowing for translation into a polyprotein that is later cleaved into functional proteins. This indicates that the gene can implement various strategies to produce proteins from a single RNA strand, which in turn minimizes the genome's overall size.
Viruses exhibiting complex transcription, utilizing subgenomic mRNAs, ribosomal frameshifting, and cleavage of polyproteins. These are all unique methods through which proteins can be synthesized from the same RNA strand.
Families that belong to this category include Coronaviridae, Flaviviridae, and Picornaviridae.
Class 5: Single-stranded RNA viruses - negative-sense
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Negative-sense RNA viruses, along with all genes classified as negative-sense, cannot be directly utilized by host ribosomes to synthesize proteins instantly. Conversely, they require transcription by viral polymerases into a "readable" complementary positive-sense form. This group can also be divided into two main categories:
Viruses that have nonsegmented genomes, where the initial phase of replication involves transcription from the negative-stranded genome driven by the viral RNA-dependent RNA polymerase to create monocistronic mRNAs corresponding to various viral proteins. A positive-sense genome copy is subsequently generated to serve as a template for creating the negative-strand genome. This replication takes place within the cytoplasm.
Viruses with segmented genomes that replicate in the cytoplasm, where the viral RNA-dependent RNA polymerase generates monocistronic mRNAs derived from each segment of the genome.
Examples of this class are represented by the families Orthomyxoviridae, Paramyxoviridae, Bunyaviridae, Filoviridae, and Rhabdoviridae, which encompasses rabies.