The sequences of oligonucleotide primers used for reverse transcription-PCR (RT-PCR) were as follows: S1-FW, 5Ј-CCGGATCCATGTTGTCCGTGTTTATT T-3Ј (BamHI site in italics); S1-RV, 5Ј-TAGTCGACTCACGTGCGCGCGCG GTGAGCA-3Ј (SalI site in italics); S2-FW, 5Ј-CCGGATCCTCTGTCAGTAC GGGTTATA-3Ј (BamHI site in italics); S2-RV, 5Ј-GGTCGACCTCAATCCTC ATGGGAGGAAAT-3Ј (SalI site in italics); Smid-FW, 5Ј-ATGGATCCGGAT TTAATGATGTGGC-3Ј (BamHI site in italics); Smid-RV, 5Ј-CCGTCGACTC ATAGAATTTCTTGTA-3Ј (SalI site in italics); N-FW, 5Ј-CTAGAATTCATG TCTTTTGTTCCTGGG-3Ј (EcoRI site in italics); and N-RV, 5Ј-GCGCTCGA GTTACACATTAGAGTCATC-3Ј (XhoI site in italics).