rule of six (viruses) rule of six (viruses) the rule of six is a feature of some paramyxovirus genomes. these rna viruses have genes made from rna and not dna, and their whole genome – that is the number of nucleotides – is always a multiple of six. this is because during their replication, these viruses are dependent on nucleoprotein molecules that each bind to six nucleotides. electron micrograph of the ribonucleoprotein of mumps virus, which has a herring-bone like structure