Intracellular viral RNAs of infected MRC-5 cells served as the source of sequence analysis. Most of the sequences of the L, M and S RNA segments of rescued rZH501-M847-A and rZH501-M847-A were determined by direct sequencing analysis of the virus-specific RT-PCR products. For sequencing of the 59-end of viral RNAs, free 59-phosphates in intracellular RNAs were removed by calf intestinal alkaline phosphatase and then the samples were treated with tobacco acid pyrophosphatase to remove the cap structure. After ligation of the RNA adaptor to uncapped RNA by using T4 RNA ligase, cDNA was synthesized by employing an RLM-RACE kit (Ambion, Austin, TX) using random primers. Amplicons were obtained by using a virusspecific and an adaptor-specific primer and subjected to sequencing analysis. For sequencing of the 39-end of viral RNAs, a poly A tail was added to viral RNAs by using a Poly(A) Polymerase Tailing Kit (Epicenter biotechnology, Madison, WI). First-strand cDNA was synthesized from RNA containing a poly A tail by using the 39Race adaptor (RLM-RACE kit, Ambion). The cDNA was subjected to PCR with an adaptor-specific primer and a virus-specific primer, and the amplicons were subjected to sequence analysis.