the primers used in this study did not carry any restriction enzyme sites at the 5terminus. Except for influenza fragment specific 2–8 nucleotides (nts), each forward primer carried 15 conserved nts containing a sequence of GGG followed by12 nts (AGCGAAAGCAGG), and each reverse primer also carried15 conserved nts containing a sequence of AA followed by 13 nts(AGTAGAAACAAGG).