There are 6 classes of viruses. The DNA viruses constitute classes I and II. The RNA viruses make up the remaining classes. Class III viruses have a double-stranded RNA genome. Class IV viruses have a positive single-stranded RNA genome, the genome itself acting as mRNA (messenger RNA. Class V viruses have a negative single-stranded RNA genome used as a template for mRNA synthesis. Class VI viruses have a positive single- stranded RNA genome but with a DNA intermediate not only in replication but also in mRNA synthesis.