The genomes of influenza A viruses, members of the Orthomyxoviridae family, consist of eight single-stranded RNA segments of negative polarity. They encode 10 proteins, since the two smallest RNAs have the genetic information for two different products by differential splicing. These RNA segments are associated with the nucleoprotein (NP) and the three P proteins (PB2, PB1, and PA) to form ribonucleoprotein (RNP) complexes (reviewed in references 23 and25). Both transcription and replication take place in the nucleus of the infected cells. The initiation of mRNA synthesis involves a cap-stealing mechanism by which cellular capped heterogeneous nuclear RNAs are used to generate primers that are elongated by the viral transcriptase.