Baltimore classification is a system used to classify viruses based on their manner of messenger RNA (mRNA) synthesis. By organizing viruses based on their manner of mRNA production, it is possible to study viruses that behave similarly as a distinct group. Seven Baltimore groups are described that take into consideration whether the viral genome is made of deoxyribonucleic acid (DNA) or ribonucleic acid (RNA), whether the genome is single- or double-stranded, and whether the sense of a single-stranded RNA genome is positive or negative. Baltimore classification also closely corresponds to the manner of replicating the genome, so Baltimore classification is useful for grouping viruses together for both transcription and replication. Certain subjects pertaining to viruses are associated with multiple, specific Baltimore groups, such as specific forms of translation of mRNA and the host range of different types of viruses. Structural characteristics such as the shape of the viral capsid, which stores the viral genome, and the evolutionary history of viruses are not necessarily related to Baltimore groups. Baltimore classification was created in 1971 by virologist David Baltimore. Since then, it has become common among virologists to use Baltimore classification alongside standard virus taxonomy, which is based on evolutionary history. In 2018 and 2019, Baltimore classification was partially integrated into virus taxonomy based on evidence that certain groups were descended from common ancestors. Various realms, kingdoms, and phyla now correspond to specific Baltimore groups. Overview Baltimore classification groups viruses together based on their manner of mRNA synthesis. Characteristics directly related to this include whether the genome is made of deoxyribonucleic acid (DNA) or ribonucleic acid (RNA), the strandedness of the genome, which can be either single- or double-stranded, and the sense of a single-stranded genome, which is either positive or negative. The primary advantage of Baltimore classification is that by classifying viruses according to the aforementioned characteristics, viruses that behave in the same manner can be studied as distinct groups. There are seven Baltimore groups numbered with Roman numerals, listed hereafter. Group I: double-stranded DNA viruses Group II: single-stranded DNA viruses Group III: double-stranded RNA viruses Group IV: positive sense single-stranded RNA viruses Group V: negative sense single-stranded RNA viruses Group VI: single-stranded RNA viruses with a DNA intermediate in their life cycle Group VII: double-stranded DNA viruses with an RNA intermediate in their life cycle Baltimore classification is chiefly based on the transcription of the viral genome, and viruses within