Total RNA was extracted from TW2575/98infected ATE cells using TRIzol TM C&T (Protech, Taiwan) according to the manufacturer's protocol. Positive-sense and negative-sense viral RNA (0.5 lg) were reverse transcribed into cDNA using SuperScript TM III reverse transcriptase (RT) (Invitrogen) with oligo dT and sense primer 5 0 -ACT GAA AAT GAT AGT GTT ATG-3 0 , respectively. PCR was performed with a proofreading DNA polymerase (KOD-Plus, Toyobo, Osaka, Japan). The PCR cycling conditions consisted of 28 cycles at 94°C for 30 s, 62°C for 30 s, and 68°C for 1 min on a PCR machine (ASTEC 818). The primer sequences for detection of the nucleocapsid gene of IBV were 5 0 -AAT GCA TCT TGG TTT CAA GC-3 0 and 5 0 -TCC TCA TCT GAG GTC AAT GC-3 0 ; for detection of GAPDH, the sequences were 5 0 -GTG AAG GTC GGT GTG AAC G3 0 and 5 0 -GGT GAA GAC ACC AGT AGA CAC TC-3 0 .