Baltimore classification of viral genomes

The Baltimore classification of viruses considers several parameters for the grouping of related viruses, based on genomic properties including:

(1) the composition of the nucleic acid (i.e. DNA or RNA);

(2) the number of strands (i.e. single- or double-stranded);

(3) the sense of single-stranded RNA genomes (i.e. positive or negative);

(4) the strategy for obtaining mRNA transcripts for gene expression. Each of the seven classes of virus, as defined by this classification scheme, shows one or more distinctions within these criteria. 

The  system does  have  its limitations (for example, ssDNA viruses are  not  further divided according to genome sense or segmentation, and  ambisense genomes are  not classed separately) but it nevertheless forms a useful consideration in the determination of virus  taxonomy.