In the same way, given S pol , the concentration of positive-sense antigenomic nucleocapsids available for binding to polymerases would be ((þ)nc -pol trc (S pol /l trc )), where (þ)nc is the total concentration of antigenomic nucleocapsids, pol trc is the concentration of the polymerases bound to the promoter region (Trc) of the anti-genomes, and l trc is the length of the promoter region. Because the anti-genome has a stronger promoter than the genome [19, 31] , which is quantified by S prom in our model, S prom -fold, more polymerases bind to the promoter of the anti-genome than to that of the genome. Under the limitation of free polymerase complex, the concentration of the polymerases newly binding to the promoters of the genomes or the anti-genomes (pol term new ) could be described as follows: 