Early studies, in which semi-quantitative hybridization techniques were used, described differential expression rates and levels of the different vRNAs. In general it appeared that the mRNAs for NS1 and NP are synthesized preferentially at the early times post infection, while the synthesis of matrix (M1) mRNA is delayed [16] [17] [18] . More recently, Vester and coworkers showed, by using quantitative RT-PCR that the vRNAs are synthesized in equimolar amounts and with similar kinetics, whereas early in infection preferential synthesis of NS1 mRNA and a delay in that of M1 mRNA was found [19] . However, how IAV temporally regulates the replication and transcription levels of its multiple genome segments is not known.