Baltimore classification

Baltimore classification is a system used to classify viruses by their routes of transferring genetic information from the genome to messenger RNA (mRNA). Seven Baltimore groups, or classes, exist and are designated by the Roman numerals I to VII. Groups are defined by whether the viral genome is composed of deoxyribonucleic acid (DNA) or ribonucleic acid (RNA); whether the genome is single- or double-stranded; and whether the virus makes DNA from RNA (reverse transcription (RT)). Viruses with single-stranded genomes are further differentiated by whether these genomes are positive-sense (+) or negative-sense (–). Viruses within each of the Baltimore groups typically share a common replication method. Additional characteristics, such as virion structure, are not directly related to Baltimore classification and may not be shared by viruses within each group.

The seven Baltimore groups are for double-stranded DNA (dsDNA) viruses, single-stranded DNA (ssDNA) viruses, double-stranded RNA (dsRNA) viruses, positive-sense single-stranded RNA (+ssRNA) viruses, negative-sense single-stranded RNA (–ssRNA) viruses, ssRNA viruses that have a DNA intermediate in their life cycle (ssRNA-RT), and dsDNA viruses that have an RNA intermediate in their life cycle (dsDNA-RT). Only one class exists for ssDNA viruses because their genomes are converted to dsDNA before transcription regardless of sense. Some viruses belong to more than one Baltimore group, such as DNA viruses that have either dsDNA or ssDNA as their genome.

Many virus characteristics do not define which Baltimore group they belong to but do correlate to specific Baltimore groups. This includes the use of RNA editing and alternative splicing, whether the virus's genome is segmented, the size and structure of the virus's genome, the host range of viruses, whether the virus packages replication and transcription machinery into virions, and unorthodox methods of translating mRNA into proteins.